Recombinational cloning using engineered recombination sites

ABSTRACT

Recombinational cloning is provided by the use of nucleic acids, vectors and methods, in vitro and in vivo, for moving or exchanging segments of DNA molecules using engineered recombination sites and recombination proteins to provide chimeric DNA molecules that have the desired characteristic(s) and/or DNA segment(s).

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation-in-part of U.S. Appl.No. 08/486,139, filed Jun. 7, 1995, which application is entirelyincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to recombinant DNA technology. DNAand vectors having engineered recombination sites are provided for usein a recombinational cloning method that enables efficient and specificrecombination of DNA segments using recombination proteins. The DNAs,vectors and methods are useful for a variety of DNA exchanges, such assubcloning of DNA, in vitro or in vivo.

[0004] 2. Related Art

[0005] Site specific recombinases. Site specific recombinases areenzymes that are present in some viruses and bacteria and have beencharacterized to have both endonuclease and ligase properties. Theserecombinases (along with associated proteins in some cases) recognizespecific sequences of bases in DNA and exchange the DNA segmentsflanking those segments. The recombinases and associated proteins arecollectively referred to as “recombination proteins” (see, e.g.,, Landy,A., Current Opinion in Biotechnology 3:699-707 (1993)).

[0006] Numerous recombination systems from various organisms have beendescribed. See, e.g., Hoess et al., Nucleic Acids Research 14(6):2287(1986); Abremski et al., J. Biol. Chem. 261(1):391 (1986); Campbell, J.Bacteriol. 174(23):7495 (1992); Qian et al., J. Biol. Chem. 267(11):7794(1992); Araki et al., J. Mol. Biol. 225(1):25 (1992); Maeser andKahnmann (1991) Mol. Gen. Genet. 230:170-176).

[0007] Many of these belong to the integrase family of recombinases(Argos et al. EMBO J. 5:433-440 (1986)). Perhaps the best studied ofthese are the Integrase/att system from bacteriophage λ (Landy, A.Current Opinions in Genetics and Devel. 3:699-707 (1993)), the Cre/loxPsystem from bacteriophage P1 (Hoess and Abremski (1990) In Nucleic Acidsand Molecular Biology, vol. 4. Eds.: Eckstein and Lilley,Berlin-Heidelberg: Springer-Verlag; pp. 90-109), and the FLP/FRT systemfrom the Saccharomyces cerevisiae 2μ circle plasmid (Broach et al. Cell29:227-234 (1982)).

[0008] While these recombination systems have been characterized forparticular organisms, the related art has only taught using recombinantDNA flanked by recombination sites, for in vivo recombination.

[0009] Backman (U.S. Pat. No. 4,673,640) discloses the in vivo use of λrecombinase to recombine a protein producing DNA segment by enzymaticsite-specific recombination using wild-type recombination sites attB andattP.

[0010] Hasan and Szybalski (Gene 56:145-151 (1987)) discloses the use ofλ Int recombinase in vivo for intramolecular recombination between wildtype attP and attB sites which flank a promoter. Because theorientations of these sites are inverted relative to each other, thiscauses an irreversible flipping of the promoter region relative to thegene of interest.

[0011] Palazzolo et al. Gene 88:25-36 (1990), discloses phage lambdavectors having bacteriophage λ arms that contain restriction sitespositioned outside a cloned DNA sequence and between wild-type loxPsites. Infection of E. coli cells that express the Cre recombinase withthese phage vectors results in recombination between the loxP sites andthe in vivo excision of the plasmid replicon, including the cloned cDNA.

[0012] Pósfai et al. (Nucl. Acids Res. 22:2392-2398 (1994)) discloses amethod for inserting into genomic DNA partial expression vectors havinga selectable marker, flanked by two wild-type FRT recognition sequences.FLP site-specific recombinase as present in the cells is used tointegrate the vectors into the genome at predetermined sites. Underconditions where the replicon is functional, this cloned genomic DNA canbe amplified.

[0013] Bebee et al. (U.S. Pat. No. 5,434,066) discloses the use ofsite-specific recombinases such as Cre for DNA containing two loxP sitesis used for in vivo recombination between the sites.

[0014] Boyd (Nucl. Acids Res. 21:817-821 (1993)) discloses a method tofacilitate the cloning of blunt-ended DNA using conditions thatencourage intermolecular ligation to a dephosphorylated vector thatcontains a wild-type loxP site acted upon by a Cre site-specificrecombinase present in E. coli host cells.

[0015] Waterhouse et al. (PCT No.93/19172 and Nucleic Acids Res. 21(9):2265 (1993)) disclose an in vivo method where light and heavy chainsof a particular antibody were cloned in different phage vectors betweenloxP and loxP 511 sites and used to transfect new E. coli cells. Cre,acting in the host cells on the two parental molecules (one plasmid, onephage), produced four products in equilibrium: two differentcointegrates (produced by recombination at either loxP or loxP 511sites), and two daughter molecules, one of which was the desiredproduct.

[0016] In contrast to the other related art, Schlake & Bode(Biochemistry 33:12746-12751 (1994)) discloses an in vivo method toexchange expression cassettes at defined chromosomal locations, eachflanked by a wild type and a spacer-mutated FRT recombination site. Adouble-reciprocal crossover was mediated in cultured mammalian cells byusing this FLP/FRT system for site-specific recombination.

[0017] Transposases. The family of enzymes, the transposases, has alsobeen used to transfer genetic information between replicons. Transposonsare structurally variable, being described as simple or compound, buttypically encode the recombinase gene flanked by DNA sequences organizedin inverted orientations. Integration of transposons can be random orhighly specific. Representatives such as Tn7, which are highlysite-specific, have been applied to the in vivo movement of DNA segmentsbetween replicons (Lucklow et al., J. Virol. 67:4566-4579 (1993)).

[0018] Devine and Boeke Nucl. Acids Res. 22:3765-3772 (1994), disclosesthe construction of artificial transposons for the insertion of DNAsegments, in vitro, into recipient DNA molecules. The system makes useof the integrase of yeast TY1 vinis-like particles. The DNA segment ofinterest is cloned, using standard methods, between the ends of thetransposon-like element TY1. In the presence of the TY1 integrase, theresulting element integrates randomly into a second target DNA molecule.

[0019] DNA cloning. The cloning of DNA segments currently occurs as adaily routine in many research labs and as a prerequisite step in manygenetic analyses. The purpose of these clonings is various, however, twogeneral purposes can be considered: (1) the initial cloning of DNA fromlarge DNA or RNA segments (chromosomes, YACs, PCR fragments, mRNA,etc.), done in a relative handful of known vectors such as pUC, pGem,pBlueScript, and (2) the subcloning of these DNA segments intospecialized vectors for functional analysis. A great deal of time andeffort is expended both in the initial cloning of DNA segments and inthe transfer of DNA segments from the initial cloning vectors to themore specialized vectors. This transfer is called subcloning.

[0020] The basic methods for cloning have been known for many years andhave changed little during that time. A typical cloning protocol is asfollows:

[0021] (1) digest the DNA of interest with one or two restrictionenzymes;

[0022] (2) gel purify the DNA segment of interest when known;

[0023] (3) prepare the vector by cutting with appropriate restrictionenzymes, treating with alaline phosphatase, gel purify etc., asappropriate;

[0024] (4) ligate the DNA segment to vector, with appropriate controlsto estimate background of uncut and self-ligated vector;

[0025] (5) introduce the resulting vector into an E. coli host cell;

[0026] (6) pick selected colonies and grow small cultures overnight;

[0027] (7) make DNA minipreps; and

[0028] (8) analyze the isolated plasmid on agarose gels (often afterdiagnostic restriction enzyme digestions) or by PCR.

[0029] The specialized vectors used for subcloning DNA segments arefunctionally diverse. These include but are not limited to: vectors forexpressing genes in various organisms; for regulating gene expression;for providing tags to aid in protein purification or to allow trackingof proteins in cells; for modifying the cloned DNA segment (e.g.,generating deletions); for the synthesis of probes (e.g., riboprobes);for the preparation of templates for DNA sequencing; for theidentification of protein coding regions; for the fusion of variousprotein-coding regions; to provide large amounts of the DNA of interest,etc. It is common that a particular investigation will involvesubcloning the DNA segment of interest into several differentspecialized vectors.

[0030] As known in the art, simple subclonings can be done in one day(e.g., the DNA segment is not large and the restriction sites arecompatible with those of the subcloning vector). However, many othersubolonings can take several weeks, especially those involving unknownsequences, long fragments, toxic genes, unsuitable placement ofrestriction sites, high backgrounds, impure enzymes, etc. Subcloning DNAfragments is thus often viewed as a chore to be done as few times aspossible.

[0031] Several methods for facilitating the cloning of DNA segments havebeen described, e.g., as in the following references.

[0032] Ferguson, J., et al. Gene 16:191 (1981), discloses a family ofvectors for subcloning fragments of yeast DNA. The vectors encodekanamycin resistance. Clones of longer yeast DNA segments can bepartially digested and ligated into the subcloning vectors. If theoriginal cloning vector conveys resistance to ampicillin, nopurification is necessary prior to transformation, since the selectionwill be for kanamycin.

[0033] Hashimoto-Gotoh, T., et al. Gene 41:125 (1986), discloses asubcloning vector with unique cloning sites within a streptomycinsensitivity gene; in a streptomycin-resistant host, only plasmids withinserts or deletions in the dominant sensitivity gene will survivestreptomycin selection.

[0034] Accordingly, traditional subcloning methods, using restrictionenzymes and ligase, are time consuming and relatively unreliable.Considerable labor is expended, and if two or more days later thedesired subclone can not be found among the candidate plasmids, theentire process must then be repeated with alternative conditionsattempted. Although site specific recombinases have been used torecombine DNA in vivo, the successful use of such enzymes in vitro wasexpected to suffer from several problems. For example, the sitespecificities and efficiencies were expected to differ in vitro;topologically-linked products were expected; and the topology of the DNAsubstrates and recombination proteins was expected to differsignificantly in vitro (see, e.g., Adams et al., J. Mol. Biol.226:661-73 (1992)). Reactions that could go on for many hours in vivowere expected to occur in significantly less time in vitro before theenzymes became inactive. Multiple DNA recombination products wereexpected in the biological host used, resulting in unsatisfactoryreliability, specificity or efficiency of subcloning. In vitrorecombination reactions were not expected to be sufficiently efficientto yield the desired levels of product.

[0035] Accordingly, there is a long felt need to provide an alternativesubcloning system that provides advantages over the known use ofrestriction enzymes and ligases.

SUMMARY OF THE INVENTION

[0036] The present invention provides nucleic acid, vectors and methodsfor obtaining chimeric nucleic acid using recombination proteins andengineered recombination sites, in vitro or in vivo. These methods arehighly specific, rapid, and less labor intensive than what is disclosedor suggested in the related background art. The improved specificity,speed and yields of the present invention facilitates DNA or RNAsubcloning, regulation or exchange useful for any related purpose. Suchpurposes include in vitro recombination of DNA segments and in vitro orin vivo insertion or modification of transcribed, replicated, isolatedor genomic DNA or RNA.

[0037] The present invention relates to nucleic acids, vectors andmethods for moving or exchanging segments of DNA using at least oneengineered recombination site and at least one recombination protein toprovide chimeric DNA molecules which have the desired characteristic()and/or DNA segment(s). Generally, one or more parent DNA molecules arerecombined to give one or more daughter molecules, at least one of whichis the desired Product DNA segment or vector. The invention thus relatesto DNA, RNA, vectors and methods to effect the exchange and/or to selectfor one or more desired products.

[0038] One embodiment of the present invention relates to a method ofmaking chimeric DNA, which comprises

[0039] (a) combining in vitro or in vivo

[0040] (i) an Insert Donor DNA molecule, comprising a desired DNAsegment flanked by a first recombination site and a second recombinationsite, wherein the first and second recombination sites do not recombinewith each other;

[0041] (ii) a Vector Donor DNA molecule containing a third recombinationsite and a fourth recombination site, wherein the third and fourthrecombination sites do not recombine with each other; and

[0042] (iii) one or more site specific recombination proteins capable ofrecombining the first and third recombinational sites and/or the secondand fourth recombinational sites;

[0043] thereby allowing recombination to occur, so as to produce atleast one Cointegrate DNA molecule, at least one desired Product DNAmolecule which comprises said desired DNA segment, and optionally aByproduct DNA molecule; and then, optionally,

[0044] (b) selecting for the Product or Byproduct DNA molecule.

[0045] Another embodiment of the present invention relates to a kitcomprising a carrier or receptacle being compartmentalized to receiveand hold therein at least one container, wherein a first containercontains a DNA molecule comprising a vector having at least tworecombination sites flanking a cloning site or a Selectable marker, asdescribed herein. The kit optionally further comprises:

[0046] (i) a second container containing a Vector Donor plasmidcomprising a subeloning vector and/or a Selectable marker of which oneor both are flanked by one or more engineered recombination sites;and/or

[0047] (ii) a third container containing at least one recombinationprotein which recognizes and is capable of recombining at least one ofsaid recombination sites.

[0048] Other embodiments include DNA and vectors useful in the methodsof the present invention. In particular, Vector Donor molecules areprovided in one embodiment, wherein DNA segments within the Vector Donorare separated either by, (i) in a circular Vector Donor, at least tworecombination sites, or (ii) in a linear Vector Donor, at least onerecombination site, where the recombination sites are preferablyengineered to enhance specificity or efficiency of recombination.

[0049] One Vector Donor embodiment comprises a first DNA segment and asecond DNA segment, the first or second segment comprising a Selectablemarker. A second Vector Donor embodiment comprises a first DNA segmentand a second DNA segment, the first or second DNA segment comprising atoxic gene. A third Vector Donor embodiment comprises a first DNAsegment and a second DNA segment, the first or second DNA segmentcomprising an inactive fragment of at least one Selectable marker,wherein the inactive fragment of the Selectable marker is capable ofreconstituting a functional Selectable marker when recombined across thefirst or second recombination site with another inactive fragment of atleast one Selectable marker.

[0050] The present recombinational cloning method possesses severaladvantages over previous in vivo methods. Since single molecules ofrecombination products can be introduced into a biological host,propagation of the desired Product DNA in the absence of other DNAmolecules (e.g., starting molecules, intermediates, and by-products) ismore readily realized. Reaction conditions can be freely adjusted invitro to optimize enzyme activities. DNA molecules can be incompatiblewith the desired biological host (e.g., YACs, genomic DNA, etc.), can beused. Recombination proteins from diverse sources can be employed,together or sequentially.

[0051] Other embodiments will be evident to those of ordinary skill inthe art from the teachings contained herein in combination with what isknown to the art

BRIEF DESCRIPTION OF THE FIGURES

[0052]FIG. 1 depicts one general method of the present invention,wherein the starting (parent) DNA molecules can be circular or linear.The goal is to exchange the new subcloning vector D for the originalcloning vector B. It is desirable in one embodiment to select for AD andagainst all the other molecules, including the Cointegrate. The squareand circle are sites of recombination: e.g., loxP sites, att sites, etc.For example, segment D can contain expression signals, new drug markers,new origins of replication, or specialized functions for mapping orsequencing DNA.

[0053]FIG. 2A depicts an in vitro method of recombining an Insert Donorplasmid (here, pEZC705) with a Vector Donor plasmid (here, pEZC726), andobtaining Product DNA and Byproduct daughter molecules. The tworecombination sites are attP and loxP on the Vector Donor. On onesegment defined by these sites is a kanamycin resistance gene whosepromoter has been replaced by the tetOP operator/promoter fromtransposon Tn10. See Sizemore et al., Nucl. Acids Res. 18(10):2875(1990). In the absence of tet repressor protein, E. coli RNA polymerasetranscribes the kanamycin resistance gene from the tetOP. If tetrepressor is present, it binds to tetOP and blocks transcription of thekanamycin resistance gene. The other segment of pEZC726 has the tetrepressor gene expressed by a constitutive promoter. Thus cellstransformed by pEZC726 are resistant to chloramphenicol, because of thechloramphenicol acetyl transferase gene on the same segment as tetR, butare sensitive to kanamycin. The recombinase-mediated reactions result inseparation of the tetR gene from the regulated kanamycin resistancegene. This separation results in kanamycin resistance in cells receivingonly the desired recombination products. The first recombinationreaction is driven by the addition of the recombinase called Integrase.The second recombination reaction is driven by adding the recombinaseCre to the Cointegrate (here, pEZC7 Cointegrate).

[0054]FIG. 2B depicts a restriction map of pEZC705.

[0055]FIG. 2C depicts a restriction map of pEZC726.

[0056]FIG. 2D depicts a restriction map of pEZC7 Cointegrate.

[0057]FIG. 2E depicts a restriction map of Intprod.

[0058]FIG. 2F depicts a restriction map of Intbypro.

[0059]FIG. 3A depicts an in vitro method of recombining an Insert Donorplasmid (here, pEZC602) with a Vector Donor plasmid (here, pEZC629), andobtaining Product (here, EZC6prod) and Byproduct (here, EZC6Bypr)daughter molecules. The two recombination sites are loxP and loxP 511.One segment of pEZC629 defined by these sites is a kanamycin resistancegene whose promoter has been replaced by the tetOP operator/promoterfrom transposon Tn10. In the absence of tet repressor protein, E. coliRNA polymerase transcribes the kanamycin resistance gene from the tetOP.If tet repressor is present, it binds to tetOP and blocks transcriptionof the kanamycin resistance gene. The other segment of pEZC629 has thetet repressor gene expressed by a constitutive promoter. Thus cellstransformed by pEZC629 are resistant to chloramphenicol, because of thechloramphenicol acetyl transferase gene on the same segment as tetR, butare sensitive to kanamycin. The reactions result in separation of thetetR gene from the regulated kanamycin resistance gene. This separationresults in kanamycin resistance in cells receiving the desiredrecombination product. The first and the second recombination events aredriven by the addition of the same recombinase, Cre.

[0060]FIG. 3B depicts a restriction map of EZC6Bypr.

[0061]FIG. 3C depicts a restriction map of EZC6prod.

[0062]FIG. 3D depicts a restriction map of pEZC602.

[0063]FIG. 3E depicts a restriction map of pEZC629.

[0064]FIG. 3F depicts a restriction map of EZC6coint.

[0065]FIG. 4A depicts an application of the in vitro method ofrecombinational cloning to subclone the chloramphenicol acetyltransferase gene into a vector for expression in eukaryotic cells. TheInsert Donor plasmid, pEZC843, is comprised of the chloramphenicolacetyl transferase gene of E. coli, cloned between loxP and aftB sitessuch that the loxP site is positioned at the 5′-end of the gene. TheVector Donor plasmid, pEZC1003, contains the cytomegalovirus eukaryoticpromoter apposed to a loxP site. The supercoiled plasmids were combinedwith lambda Integrase and Cre recombinase in vitro. After incubation,competent E. coli cells were transformed with the recombinationalreaction solution. Aliquots of transformations were spread on agarplates containing kanamycin to select for the Product molecule (hereCMVProd).

[0066]FIG. 4B depicts a restriction map of pEZC843.

[0067]FIG. 4C depicts a restriction map of pEZC1003.

[0068]FIG. 4D depicts a restriction map of CMVBypro.

[0069]FIG. 4E depicts a restriction map of CMVProd.

[0070]FIG. 4F depicts a restriction map of CMVcoint.

[0071]FIG. 5A depicts a vector diagram of pEZC1301.

[0072]FIG. 5B depicts a vector diagram of pEZC1305.

[0073]FIG. 5C depicts a vector diagram of pEZC1309.

[0074]FIG. 5D depicts a vector diagram of pEZC1313.

[0075]FIG. 5E depicts a vector diagram of pEZC1317.

[0076]FIG. 5F depicts a vector diagram of pEZC1321.

[0077]FIG. 5G depicts a vector diagram of pEZC1405.

[0078]FIG. 5H depicts a vector diagram of pEZC1502.

[0079]FIG. 6A depicts a vector diagram of pEZC1603.

[0080]FIG. 6B depicts a vector diagram of pEZC1706.

[0081]FIG. 7A depicts a vector diagram of pEZC2901.

[0082]FIG. 7B depicts a vector diagram of pEZC2913

[0083]FIG. 7C depicts a vector diagram of pEZC3101.

[0084]FIG. 7D depicts a vector diagram of pEZC1802.

[0085]FIG. 8A depicts a vector diagram of pGEX-2TK.

[0086]FIG. 8B depicts a vector diagram of pEZC3501.

[0087]FIG. 8C depicts a vector diagram of pEZC3601.

[0088]FIG. 8D depicts a vector diagram of pEZC3609.

[0089]FIG. 8E depicts a vector diagram of pEZC3617.

[0090]FIG. 8F depicts a vector diagram of pEZC3606.

[0091]FIG. 8G depicts a vector diagram of pEZC3613.

[0092]FIG. 8H depicts a vector diagram of pEZC3621.

[0093]FIG. 8I depicts a vector diagram of GST-CAT.

[0094]FIG. 8J depicts a vector diagram of GST-phoA.

[0095]FIG. 8K depicts a vector diagram of pEZC3201.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0096] It is unexpectedly discovered in the present invention thatsubcloning reactions can be provided using recombinational cloning.Recombination cloning according to the present invention uses DNAs,vectors and methods, in vitro and in vivo, for moving or exchangingsegments of DNA molecules using engineered recombination sites andrecombination proteins. These methods provide chimeric DNA moleculesthat have the desired characteristic(s) and/or DNA segment(s).

[0097] The present invention thus provides nucleic acid, vectors andmethods for obtaining chimeric nucleic acid using recombination proteinsand engineered recombination sites, in vitro or in vivo. These methodsare highly specific, rapid, and less labor intensive than what isdisclosed or suggested in the related background art. The improvedspecificity, speed and yields of the present invention facilitates DNAor RNA subcloning, regulation or exchange useful for any relatedpurpose. Such purposes include in vitro recombination of DNA segmentsand in vitro or in vivo insertion or modification of transcribed,replicated, isolated or genomic DNA or RNA.

Definitions

[0098] In the description that follows, a number of terms used inrecombinant DNA technology are utilized extensively. In order to providea clear and consistent understanding of the specification and claims,including the scope to be given such terms, the following definitionsare provided.

[0099] Byproduct: is a daughter molecule (a new clone produced after thesecond recombination event during the recombinational cloning process)lacking the DNA which is desired to be subcloned.

[0100] Cointegrate: is at least one recombination intermediate DNAmolecule of the present invention that contains both parental (starting)DNA molecules. It will usually be circular. In some embodiments it canbe linear.

[0101] Host: is any prokaryotic or eukaryotic organism that can be arecipient of the recombinational cloning Product. A “host,” as the termis used herein, includes prokaryotic or eukaryotic organisms that can begenetically engineered. For examples of such hosts, see Maniatis et al.,Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory,Cold Spring Harbor, New York (1982).

[0102] Insert: is the desired DNA segment (segment A of FIG. 1) whichone wishes to manipulate by the method of the present invention. Theinsert can have one or more genes.

[0103] Insert Donor: is one of the two parental DNA molecules of thepresent invention which carries the Insert. The Insert Donor DNAmolecule comprises the Insert flanked on both sides with recombinationsignals. The Insert Donor can be linear or circular. In one embodimentof the invention, the Insert Donor is a circular DNA molecule andfurther comprises a cloning vector sequence outside of the recombinationsignals (see FIG. 1).

[0104] Product: is one or both the desired daughter molecules comprisingthe A and D or B and C sequences which are produced after the secondrecombination event during the recombinational cloning process (see FIG.1). The Product contains the DNA which was to be cloned or subcloned.

[0105] Promoter: is a DNA sequence generally described as the 5′-regionof a gene, located proximal to the start codon. The transcription of anadjacent DNA segment is initiated at the promoter region. A repressiblepromoter's rate of transcription decreases in response to a repressingagent. An inducible promoter's rate of transcription increases inresponse to an inducing agent. A constitutive promoter's rate oftranscription is not specifically regulated, though it can vary underthe influence of general metabolic conditions.

[0106] Recognition sequence: Recognition sequences are particular DNAsequences which a protein, DNA, or RNA molecule (e.g., restrictionendonuclease, a modification methylase, or a recombinase) recognizes andbinds. For example, the recognition sequence for Cre recombinase is loxPwhich is a 34 base pair sequence comprised of two 13 base pair invertedrepeats (serving as the recombinase binding sites) flanking an 8 basepair core sequence. See FIG. 1 of Sauer, B., Current Opinion inBiotechnology 5:521-527 (1994). Other examples of recognition sequencesare the attB, attP, attL, and attR sequences which are recognized by therecombinase enzyme λ Integrase. attB is an approximately 25 base pairsequence containing two 9 base pair core-type Int binding sites and a 7base pair overlap region. attP is an approximately 240 base pairsequence containing core-type Int binding sites and arm-type Int bindingsites as well as sites for auxiliary proteins IHF, FIS, and Xis. SeeLandy, Current Opinion in Biotechnology 3:699-707 (1993). Such sites arealso engineered according to the present invention to enhance methodsand products.

[0107] Recombinase: is an enzyme which catalyzes the exchange of DNAsegments at specific recombination sites.

[0108] Recombinational Cloning: is a method described herein, wherebysegments of DNA molecules are exchanged, inserted, replaced, substitutedor modified, in vitro or in vivo.

[0109] Recombination proteins: include excisive or integrative proteins,enzymes, co-factors or associated proteins that are involved inrecombination reactions involving one or more recombination sites. See,Landy (1994), infra.

[0110] Repression cassette: is a DNA segment that contains a repressorof a Selectable marker present in the subcloning vector.

[0111] Selectable marker: is a DNA segment that allows one to select foror against a molecule or a cell that contains it, often under particularconditions. These markers can encode an activity, such as, but notlimited to, production of RNA, peptide, or protein, or can provide abinding site for RNA, peptides, proteins, inorganic and organiccompounds or compositions and the like. Examples of Selectable markersinclude but are not limited to: (1) DNA segments that encode productswhich provide resistance against otherwise toxic compounds (e.g.,antibiotics); (2) DNA segments that encode products which are otherwiselacking in the recipient cell (e.g., tRNA genes, auxotrophic markers);(3) DNA segments that encode products which suppress the activity of agene product; (4) DNA segments that encode products which can be readilyidentified (e.g., phenotypic markers such as β-galactosidase, greenfluorescent protein (GFP), and cell surface proteins); (5) DNA segmentsthat bind products which are otherwise detrimental to cell survivaland/or function; (6) DNA segments that otherwise inhibit the activity ofany of the DNA segments described in Nos. 1-5 above (e.g., antisenseoligonucleotides); (7) DNA segments that bind products that modify asubstrate (e.g. restriction endonucleases); (8) DNA segments that can beused to isolate a desired molecule (e.g. specific protein bindingsites); (9) DNA segments that encode a specific nucleotide sequencewhich can be otherwise non-functional (e.g., for PCR amplification ofsubpopulations of molecules); and/or (10) DNA segments, which whenabsent, directly or indirectly confer sensitivity to particularcompounds.

[0112] Selection scheme: is any method which allows selection,enrichment, or identification of a desired Product or Product(s) from amixture containing the Insert Donor, Vector Donor, and/or anyintermediates, (e.g a Cointegrate) Byproducts. The selection schemes ofone preferred embodiment have at least two components that are eitherlinked or unlinked during recombinational cloning. One component is aSelectable marker. The other component controls the expression in vitroor in vivo of the Selectable marker, or survival of the cell harboringthe plasmid carrying the Selectable marker. Generally, this controllingelement will be a repressor or inducer of the Selectable marker, butother means for controlling expression of the Selectable marker can beused. Whether a repressor or activator is used will depend on whetherthe marker is for a positive or negative selection, and the exactarrangement of the various DNA segments, as will be readily apparent tothose skilled in the art. A preferred requirement is that the selectionscheme results in selection of or enrichment for only one or moredesired Products. As defined herein, to select for a DNA moleculeincludes (a) selecting or enriching for the presence of the desired DNAmolecule, and (b) selecting or enriching against the presence of DNAmolecules that are not the desired DNA molecule.

[0113] In one embodiment, the selection schemes (which can be carriedout reversed) will take one of three forms, which will be discussed interms of FIG. 1. The first, exemplified herein with a Selectable markerand a repressor therefor, selects for molecules having segment D andlacking segment C. The second selects against molecules having segment Cand for molecules having segment D. Possible embodiments of the secondform would have a DNA segment carrying a gene toxic to cells into whichthe in vitro reaction products are to be introduced. A toxic gene can bea DNA that is expressed as a toxic gene product (a toxic protein orRNA), or can be toxic in and of itself. (In the latter case, the toxicgene is understood to carry its classical definition of “heritabletrait”.)

[0114] Examples of such toxic gene products are well known in the art,and include, but are not limited to, restriction endonucleases (e.g.,DpnI) and genes that kill hosts in the absence of a suppressingfunction, e.g. kicB. A toxic gene can alternatively be selectable invitro, e.g., a restriction site.

[0115] In the second form, segment D carries a Selectable marker. Thetoxic gene would eliminate transformants harboring the Vector Donor,Cointegrate, and Byproduct molecules, while the Selectable marker can beused to select for cells containing the Product and against cellsharboring only the Insert Donor.

[0116] The third form selects for cells that have both segments A and Din cis on the same molecule, but not for cells that have both segmentsin trans on different molecules. This could be embodied by a Selectablemarker that is split into two inactive fragments, one each on segments Aand D.

[0117] The fragments are so arranged relative to the recombination sitesthat when the segments are brought together by the recombination event,they reconstitute a functional Selectable marker. For example, therecombinational event can link a promoter with a structural gene, canlink two fragments of a structural gene, or can link genes that encode aheterodimeric gene product needed for survival, or can link portions ofa replicon.

[0118] Site-specific recombinase: is a type of recombinase whichtypically has at least the following four activities: (1) recognition ofone or two specific DNA sequences; (2) cleavage of said DNA sequence orsequences; (3) DNA topoisomerase activity involved in strand exchange;and (4) DNA ligase activity to reseal the cleaved strands of DNA. SeeSauer, B., Current Opinions in Biotechnology 5:521-527 (1994).Conservative site-specific recombination is distinguished fromhomologous recombination and transposition by a high degree ofspecificity for both partners. The strand exchange mechanism involvesthe cleavage and rejoining of specific DNA sequences in the absence ofDNA synthesis (Landy, A. (1989) Ann. Rev. Biochem. 58:913-949).

[0119] Subcloning vector: is a cloning vector comprising a circular orlinear DNA molecule which includes an appropriate replicon. In thepresent invention, the subcloning vector (segment D in FIG. 1) can alsocontain functional and/or regulatory elements that are desired to beincorporated into the final product to act upon or with the cloned DNAInsert (segment A in FIG. 1). The subcloning vector can also contain aSelectable marker (contained in segment C in FIG. 1).

[0120] Vector: is a DNA that provides a useful biological or biochemicalproperty to an Insert. Examples include plasmids, phages, and other DNAsequences which are able to replicate or be replicated in vitro or in ahost cell, or to convey a desired DNA segment to a desired locationwithin a host cell. A Vector can have one or more restrictionendonuclease recognition sites at which the DNA sequences can be cut ina determinable fashion without loss of an essential biological functionof the vector, and into which a DNA fragment can be spliced in order tobring about its replication and cloning. Vectors can further provideprimer sites, e.g., for PCR, transcriptional and/or translationalinitiation and/or regulation sites, recombinational signals, replicons,Selectable markers, etc. Clearly, methods of inserting a desired DNAfragment which do not require the use of homologous recombination orrestriction enzymes (such as, but not limited to, UDG cloning of PCRfragments (U.S. Pat. No. 5,334,575, entirely incorporated herein byreference), T:A cloning, and the like) can also be applied to clone afragment of DNA into a cloning vector to be used according to thepresent invention. The cloning vector can further contain a Selectablemarker suitable for use in the identification of cells transformed withthe cloning vector.

[0121] Vector Donor: is one of the two parental DNA molecules of thepresent invention which carries the DNA segments encoding the DNA vectorwhich is to become part of the desired Product. The Vector Donorcomprises a subeloning vector D (or it can be called the cloning vectorif the Insert Donor does not already contain a cloning vector) and asegment C flanked by recombination sites (see FIG. 1). Segments C and/orD can contain elements that contribute to selection for the desiredProduct daughter molecule, as described above for selection schemes. Therecombination signals can be the same or different, and can be actedupon by the same or different recombinases. In addition, the VectorDonor can be linear or circular.

Description

[0122] One general scheme for an in vitro or in vivo method of theinvention is shown in FIG. 1, where the Insert Donor and the VectorDonor can be either circular or linear DNA, but is shown as circular.Vector D is exchanged for the original cloning vector A. It is desirableto select for the daughter vector containing elements A and D andagainst other molecules, including one or more Cointegrate(s). Thesquare and circle are different sets of recombination sites (e.g., loxsites or att sites). Segment A or D can contain at least one SelectionMarker, expression signals, origins of replication, or specializedfunctions for detecting, selecting, expressing, mapping or sequencingDNA, where D is used in this example.

[0123] Examples of desired DNA segments that can be part of Element A orD include, but are not limited to, PCR products, large DNA segments,genomic clones or fragments, cDNA clones, functional elements, etc., andgenes or partial genes, which encode useful nucleic acids or proteins.Moreover, the recombinational cloning of the present invention can beused to make ex vivo and in vivo gene transfer vehicles for proteinexpression and/or gene therapy.

[0124] In FIG. 1, the scheme provides the desired Product as containingvectors D and A, as follows. The Insert Donor (containing A and B) isfirst recombined at the square recombination sites by recombinationproteins, with the Vector Donor (containing C and D), to form aCo-integrate having each of A-D-C-B. Next, recombination occurs at thecircle recombination sites to form Product DNA (A and D) and ByproductDNA (C and B). However, if desired, two or more different Co-integratescan be formed to generate two or more Products.

[0125] In one embodiment of the present in vitro or in vivorecombinational cloning method, a method for selecting at least onedesired Product DNA is provided. This can be understood by considerationof the map of plasmid pEZC726 depicted in FIG. 2. The two exemplaryrecombination sites are attP and loxP. On one segment defined by thesesites is a kanamycin resistance gene whose promoter has been replaced bythe tetOP operator/promoter from transposon Tn10. In the absence of tetrepressor protein, E. coli RNA polymerase transcribes the kanamycinresistance gene from the tetOP. If tet repressor is present, it binds totetOP and blocks transcription of the kanamycin resistance gene. Theother segment of pEZC726 has the tet repressor gene expressed by aconstitutive promoter. Thus cells transformed by pEZC726 are resistantto chloramphenicol, because of the chloramphenicol acetyl transferasegene on the same segment as tetR, but are sensitive to kanamycin. Therecombination reactions result in separation of the tetR gene from theregulated kanamycin resistance gene. This separation results inkanamycin resistance in cells receiving the desired recombinationProduct.

[0126] Two different sets of plasmids were constructed to demonstratethe in vitro method. One set, for use with Cre recombinase only (cloningvector 602 and subcloning vector 629 (FIG. 3)) contained loxP and loxP511 sites. A second set, for use with Cre and integrase (cloning vector705 and subcloning vector 726 (FIG. 2)) contained loxP and att sites.The efficiency of production of the desired daughter plasmid was about60 fold higher using both enzymes than using Cre alone. Nineteen oftwenty four colonies from the Cre-only reaction contained the desiredproduct, while thirty eight of thir eight colonies from the integraseplus Cre reaction contained the desired product plasmid.

[0127] Other Selection Schemes A variety of selection schemes can beused that are known in the art as they can suit a particular purpose forwhich the recombinational cloning is carried out Depending uponindividual preferences and needs, a number of different types ofselection schemes can be used in the recombinational cloning method ofthe present invention. The skilled artisan can take advantage of theavailability of the many DNA segments or methods for making them and thedifferent methods of selection that are routinely used in the art. SuchDNA segments include but are not limited to those which encodes anactivity such as, but not limited to, production of RNA, peptide, orprotein, or providing a binding site for such RNA, peptide, or protein.Examples of DNA molecules used in devising a selection scheme are givenabove, under the definition of “selection scheme”

[0128] Additional examples include but are not limited to:

[0129] (i) Generation of new primer sites for PCR (e.g., juxtapositionof two DNA sequences that were not previously juxtaposed);

[0130] (ii) Inclusion of a DNA sequence acted upon by a restrictionendonuclease or other DNA modifying enzyme, chemical, ribozyme, etc.;

[0131] (iii) Inclusion of a DNA sequence recognized by a DNA bindingprotein, RNA, DNA, chemical, etc.) (e.g., for use as an affinity tag forselecting for or excluding from a population) (Davis, Nucl. Acids Res.24:702-706 (1996); J. Virol. 69: 8027-8034 (1995));

[0132] (iv) In vitro selection of RNA ligands for the ribosomal L22protein associated with Epstein-Barr virus-expressed RNA by usingrandomized and cDNA-derived RNA libraries;

[0133] (vi) The positioning of functional elements whose activityrequires a specific orientation or juxtaposition (e.g., (a) arecombination site which reacts poorly in trans, but when placed in cis,in the presence of the appropriate proteins, results in recombinationthat destroys certain populations of molecules; (e.g., reconstitution ofa promoter sequence that allows in vitro RNA synthesis). The RNA can beused directly, or can be reverse transcribed to obtain the desired DNAconstruct;

[0134] (vii) Selection of the desired product by size (e.g.,fractionation) or other physical property of the molecule(s); and

[0135] (viii) Inclusion of a DNA sequence required for a specificmodification (e.g., methylation) that allows its identification.

[0136] After formation of the Product and Byproduct in the method of thepresent invention, the selection step can be carried out either in vitroor in vivo depending upon the particular selection scheme which has beenoptionally devised in the particular recombinational cloning procedure.

[0137] For example, an in vitro method of selection can be devised forthe Insert Donor and Vector Donor DNA molecules. Such scheme can involveengineering a rare restriction site in the starting circular vectors insuch a way that after the recombination events the rare cutting sitesend up in the Byproduct. Hence, when the restriction enzyme which bindsand cuts at the rare restriction site is added to the reaction mixturein vitro, all of the DNA molecules carrying the rare cutting site, i.e.,the starting DNA molecules, the Cointegrate, and the Byproduct, will becut and rendered nonreplicable in the intended host cell. For example,cutting sites in segments B and C (see FIG. 1) can be used to selectagainst all molecules except the Product. Alternatively, only a cuttingsite in C is needed if one is able to select for segment D, e.g., by adrug resistance gene not found on B.

[0138] Similarly, an in vitro selection method can be devised whendealing with linear DNA molecules. DNA sequences complementary to a PCRprimer sequence can be so engineered that they are transferred, throughthe recombinational cloning method, only to the Product molecule. Afterthe reactions are completed, the appropriate primers are added to thereaction solution and the sample is subjected to PCR. Hence, all or partof the Product molecule is amplified.

[0139] Other in vivo selection schemes can be used with a variety of E.coli cell lines. One is to put a repressor gene on one segment of thesubeloning plasmid, and a drug marker controlled by that repressor onthe other segment of the same plasmid. Another is to put a killer geneon segment C of the subcloning plasmid (FIG. 1). Of course a way mustexist for growing such a plasmid, i.e., there must exist circumstancesunder which the killer gene will not kill. There are a number of thesegenes known which require particular strains of E. coli. One such schemeis to use the restriction enzyme DpnI, which will not cleave unless itsrecognition sequence GATC is methylated. Many popular common E. colistrains methylate GATC sequences, but there are mutants in which clonedDpnI can be expressed without harm.

[0140] Of course analogous selection schemes can be devised for otherhost organisms. For example, the tet repressor/operator of Tn10 has beenadapted to control gene expression in eukaryotes (Gossen, M., andBujard,, H., Proc. Natl. Acad. Sci. USA 89:5547-5551 (1992)). Thus thesame control of drug resistance by the tet repressor exemplified hereincan be applied to select for Product in eukaryotic cells.

Recombination Proteins

[0141] In the present invention, the exchange of DNA segments isachieved by the use of recombination proteins, including recombinasesand associated co-factors and proteins. Various recombination proteinsare described in the art. Examples of such recombinases include:

[0142] Cre: A protein from bacteriophage P1 (Abremski and Hoess, J.Biol. Chem. 259(3):1509-1514 (1984)) catalyzes the exchange (i.e.,causes recombination) between 34 bp DNA sequences called loxP (locus ofcrossover) sites (See Hoess et al., Nucl. Acids Res. 14(5):2287 (1986)).Cre is available commercially (Novagen, Catalog No. 69247-1).Recombination mediated by Cre is freely reversible. From thermodynamicconsiderations it is not surprising that Cre-mediated integration(recombination between two molecules to form one molecule) is much lessefficient than Cre-mediated excision (recombination between two loxPsites in the same molecule to form two daughter molecules). Cre works insimple buffers with either magnesium or spermidine as a cofactor, as iswell known in the arL The DNA substrates can be either linear orsupercoiled. A number of mutant loxP sites have been described (Hoess etal., supra). One of these, loxP 511, recombines with another loxP 511site, but will not recombine with a loxP site.

[0143] Integrase: A protein from bacteriophage lambda that mediates theintegration of the lambda genome into the E. coli chromosome. Thebacteriophage λ Int recombinational proteins promote irreversiblerecombination between its substrate att sites as part of the formationor induction of a lysogenic state. Reversibility of the recombinationreactions results from two independent pathways for integrative andexcisive recombination. Each pathway uses a unique, but overlapping, setof the 15 protein binding sites that comprise att site DNAs. Cooperativeand competitive interactions involving four proteins (Int, Xis, IHF andFIS) determine the direction of recombination.

[0144] Integrative recombination involves the Int and IHF proteins andsites attP (240 bp) and attB (25 bp). Recombination results in theformation of two new sites: attL and attR. Excisive recombinationrequires Int IHF, and Xis, and sites attL and attR to generate attP andattB. Under certain conditions, FIS stimulates excisive recombination.In addition to these normal reactions, it should be appreciated thatattP and attB, when placed on the same molecule, can promote excisiverecombination to generate two excision products, one with attL and onewith attR. Similarly, intermolecular recombination between moleculescontaining attL and attR, in the presence of Int, IHF and Xis, canresult in integrative recombination and the generation attP and attB.Hence, by flanking DNA segments with appropriate combinations ofengineered att sites, in the presence of the appropriate recombinationproteins, one can direct excisive or integrative recombination, asreverse reactions of each other.

[0145] Each of the att sites contains a 15 bp core sequence; individualsequence elements of functional significance lie within, outside, andacross the boundaries of this common core (Landy, A., Ann. Rev. Biochem.58:913 (1989)). Efficient recombination between the various att sitesrequires that the sequence of the central common region be identicalbetween the recombining partners, however, the exact sequence is nowfound to be modifiable. Consequently, derivatives of the att site withchanges within the core are now discovered to recombine as least asefficiently as the native core sequences.

[0146] Integrase acts to recombine the attP site on bacteriophage lambda(about 240 bp) with the attB site on the E. coli genome (about 25 bp)(Weisberg, R. A. and Landy, A. in Lambda II, p. 211 (1983), Cold SpringHarbor Laboratory)), to produce the integrated lambda genome flanked byattL (about 100 bp) and attR (about 160 bp) sites. In the absence of Xis(see below), this reaction is essentially irreversible. The integrationreaction mediated by integrase and IHF works in vitro, with simplebuffer containing spermidine. Integrase can be obtained as described byNash, H. A., Methods of Enzymology 100:210-216 (1983). IHF can beobtained as described by Filutowicz, M., et al., Gene 147:149-150(1994).

[0147] In the presence of the λ protein Xis (excise) integrase catalyzesthe reaction of attR and attL to form attP and attB, i.e., it promotesthe reverse of the reaction described above. This reaction can also beapplied in the present invention.

[0148] Other Recombination Systens. Numerous recombination systems fromvarious organisms can also be used, based on the teaching and guidanceprovided herein. See, e.g., Hoess et al., Nucleic Acids Research14(6):2287 (1986); Abremski et al., J. Biol. Chem.261(1):391 (1986);Campbell, J. Bacteriol. 174(23):7495 (1992); Qian et al., J. Biol. Chem.267(11):7794 (1992); Araki et al., J. Mol. Biol. 225(l):25 (1992)). Manyof these belong to the integrase family of recombinases (Argos et al.EMBO J. 5:433440 (1986)). Perhaps the best studied of these are theIntegrase/att system from bacteriophage λ (Landy, A. (1993) CurrentOpinions in Genetics and Devel. 3:699-707), the Cre/loxP system frombacteriophage P1 (Hoess and Abremski (1990) In Nucleic Acids andMolecular Biology, vol. 4. Eds.: Eckstein and Lilley, Berlin-Heidelberg:Springer-Verlag; pp. 90-109), and the FLP/FRT system from theSaccharomyces cerevisiae 2μ circle plasmid (Broach et al. Cell29:227-234 (1982)).

[0149] Members of a second family of site-specific recombinases, theresolvase family (e.g. γδ, Tn3 resolvase, Hin, Gin, and Cin) are alsoknown. Members of this highly related family of recombinases aretypically constrained to intramolecular reactions (e.g., inversions andexcisions) and can require host-encoded factors. Mutants have beenisolated that relieve some of the requirements for host factors (Maeserand Kahnmann (1991) Mol. Gen. Genet. 230:170-176), as well as some ofthe constraints of intramolecular recombination.

[0150] Other site-specific recombinases similar to λ Int and similar toP1 Cre can be substituted for Int and Cre. Such recombinases are known.In many cases the purification of such other recombinases has beendescribed in the art. In cases when they are not known, cell extractscan be used or the enzymes can be partially purified using proceduresdescribed for Cre and Int.

[0151] While Cre and Int are described in detail for reasons of example,many related recombinase systems exist and their application to thedescribed invention is also provided according to the present invention.The integrase family of site-specific recombinases can be used toprovide alternative recombination proteins and recombination sites forthe present invention, as site-specific recombination proteins encodedby bacteriophage lambda, phi 80, P22, P2, 186, P4 and P1. This group ofproteins exhibits an unexpectedly large diversity of sequences. Despitethis diversity, all of the recombinases can be aligned in theirC-terminal halves.

[0152] A 40-residue region near the C terminus is particularly wellconserved in all the proteins and is homologous to a region near the Cterminus of the yeast 2 mu plasmid Flp protein. Three positions areperfectly conserved within this family: histidine, arginine and tyrosineare found at respective alignment positions 396, 399 and 433 within thewell-conserved C-terminal region. These residues contribute to theactive site of this family of recombinases, and suggest that tyrosine433forns a transient covalent linkage to DNA during strand cleavage andrejoining. See, e.g., Argos, P. et al., EMBO J. 5:433-40 (1986).

[0153] Alternatively, IS231 and other Bacillus thuringiensistransposable elements could be used as recombination proteins andrecombination sites. Bacillus thuringiensis is an entomopathogenicbacterium whose toxicity is due to the presence in the sporangia ofdelta-endotoxin crystals active against agricultural pests and vectorsof human and animal diseases. Most of the genes coding for these toxinproteins are plasmid-borne and are generally structurally associatedwith insertion sequences (IS231, IS232, IS240, ISBT1 and ISBT2) andtransposons (Tn4430 and Tn5401). Several of these mobile elements havebeen shown to be active and participate in the crystal gene mobility,thereby contributing to the variation of bacterial toxicity.

[0154] Structural analysis of the iso-IS231 elements indicates that theyare related to IS1151 from Clostridium perfringens and distantly relatedto IS4 and IS186 from Escherichia coli. Like the other IS4 familymembers, they contain a conserved transposase-integrase motif found inother IS families and retroviruses.

[0155] Moreover, functional data gathered from IS231A in Escherichiacoli indicate a non-replicative mode of transposition; with a preferencefor specific targets. Similar results were also obtained in Bacillussubtilis and B. thuringiensis. See, e.g., Mahillon, J. et al., Genetica93:13-26 (1994); Campbell, J. Bacteriol. 7495-7499 (1992).

[0156] The amount of recombinase which is added to drive therecombination reaction can be determined by using known assays.Specifically, titration assay is used to determine the appropriateamount of a purified recombinase enzyme, or the appropriate amount of anextract.

[0157] Engineered Recombination Sites. The above recombinases andcorresponding recombinase sites are suitable for use in recombinationcloning according to the present invention. However, wild-typerecombination sites contain sequences that reduce the efficiency orspecificity of recombination reactions as applied in methods of thepresent invention. For example, multiple stop codons in attB, attR,attP, attL and loxP recombination sites occur in multiple reading frameson both strands, so recombination efficiencies are reducted, e.g., wherethe coding sequence must cross the recombination sites, (only onereading frame is available on each strand of loxP and attB sites) orimpossible (in attP, attR or attL).

[0158] Accordingly, the present invention also provides engineeredrecombination sites that overcome these problems. For example, att sitescan be engineered to have one or multiple mutations to enhancespecificity or efficiency of the recombination reaction and theproperties of Product DNAs (e.g., att1, att2, and att3 sites); todecrease reverse reaction (e.g., removing P1 and H1 from attB). Thetesting of these mutants determines which mutants yield sufficientrecombinational activity to be suitable for recombination subcloningaccording to the present invention.

[0159] Mutations can therefore be introduced into recombination sitesfor enhancing site specific recombination. Such mutations include, butare not limited to: recombination sites without translation stop codonsthat allow fusion proteins to be encoded; recombination sites recognizedby the same proteins but differing in base sequence such that they reactlargely or exclusively with their homologous partners allow multiplereactions to be contemplated. Which particular reactions take place canbe specified by which particular partners are present in the reactionmixture. For example, a tripartite protein fusion could be accomplishedwith parental plasmids containing recombination sites attR1 and attR2;attL1 and attL3; and/or attR3 and attL2.

[0160] There are well known procedures for introducing specificmutations into nucleic acid sequences. A number of these are describedin Ausubel, F. M. et al., Current Protocols in Molecular Biology, WileyInterscience, New York (1989-1996). Mutations can be designed intooligonucleotides, which can be used to modify existing cloned sequences,or in amplification reactions. Random mutagenesis can also be employedif appropriate selection methods are available to isolate the desiredmutant DNA or RNA. The presence of the desired mutations can beconfirmed by sequencing the nucleic acid by well known methods.

[0161] The following non-limiting methods can be used to engineer a coreregion of a given recombination site to provide mutated sites suitablefor use in the present invention:

[0162] 1. By recombination of two parental DNA sequences bysite-specific (e.g. attL and attR to give attB) or other (e.g.homologous) recombination mechanisms. The DNA parental DNA segmentscontaining one or more base alterations resulting in the final coresequence;

[0163] 2. By mutation or mutagenesis (site-specific, PCR, random,spontaneous, etc) directly of the desired core sequence;

[0164] 3. By mutagenesis (site-specific, PCR, random, spontanteous, etc)of parental DNA sequences, which are recombined to generate a desiredcore sequence; and

[0165] 4. By reverse transcription of an RNA encoding the desired coresequence.

[0166] The functionality of the mutant recombination sites can bedemonstrated in ways that depend on the particular characteristic thatis desired. For example, the lack of translation stop codons in arecombination site can be demonstrated by expressing the appropriatefusion proteins. Specificity of recombination between homologouspartners can be demonstrated by introducing the appropriate moleculesinto in vitro reactions, and assaying for recombination products asdescribed herein or known in the art. Other desired mutations inrecombination sites might include the presence or absence of restrictionsites, translation or transcription start signals, protein bindingsites, and other known functionalities of nucleic acid base sequences.Genetic selection schemes for particular functional attributes in therecombination sites can be used according to known method steps. Forexample, the modification of sites to provide (from a pair of sites thatdo not interact) partners that do interact could be achieved byrequiring deletion, via recombination between the sites, of a DNAsequence encoding a toxic substance. Similarly, selection for sites thatremove translation stop sequences, the presence or absence of proteinbinding sites, etc., can be easily devised by those skilled in the art.

[0167] Accordingly, the present invention provides a nucleic acidmolecule, comprising at least one DNA segment having at least twoengineered recombination sites flanking a Selectable marker and/or adesired DNA segment, wherein at least one of said recombination sitescomprises a core region having at least one engineered mutation thatenhances recombination in vitro in the formation of a Cointegrate DNA ora Product DNA.

[0168] The nucleic acid molecule can have at least one mutation thatconfers at least one enhancement of said recombination, said enhancementselected from the group consisting of substantially (i) favoringexcisive integration; (ii) favoring excisive recombination; (ii)relieving the requirement for host factors; (iii) increasing theefficiency of said Cointegrate DNA or Product DNA formation; and (iv)increasing the specificity of said Cointegrate DNA or Product DNAformation.

[0169] The nucleic acid molecule preferably comprises at least onerecombination site derived from attB, attP, attL or attR. Morepreferably the att site is selected from att1, att2, or att3, asdescribed herein.

[0170] In a preferred embodiment, the core region comprises a DNAsequence selected from the group consisting of: (a)RKYCWGCTTTYKTRTACNAASTSGB(m-att); (SEQ ID NO:1) (b)AGCCWGCTTTYKTRTACNAACTSGB(m-attB); (SEQ ID NO:2) (c)GTTCAGCTTTCKTRTACNAACTSGB(m-attR); (SEQ ID NO:3) (d)AGCCWGCTTTCKTRTACNAAGTSGB(m-attL); (SEQ ID NO:4) (e)GTTCAGCTTTYKTRTACNAAGTSGB(m-attp1); (SEQ ID NO:5)

[0171] or a corresponding or complementary DNA or RNA sequence, whereinR=A or G; K=G or T/U; Y=C or T/U; W=A or T/U; N=A or C or G or T/U;S=Cor G; and B=C or G or T/U, as presented in 37 C.F.R §1.822, which isentirely incorporated herein by reference, wherein the core region doesnot contain a stop codon in one or more reading frames.

[0172] The core region also preferably comprises a DNA sequence selectedfrom the group consisting of: (a) AGCCTGCTTTTTTGTACAAACTTGT(attB1); (SEQID NO:6) (b) AGCCTGCTTTCTTGTACAAACTTGT(attB2); (SEQ ID NO:7) (c)ACCCAGCTTTCTTGTACAAACTTGT(attB3); (SEQ ID NO:8) (d)GTTCAGCTTTTTTGTACAAACTTGT(attR1); (SEQ ID NO:9) (e)GTTCAGCTTTCTTGTACAAACTTGT(attR2); (SEQ ID NO:10) (f)GTTCAGCTTTCTTGTACAAAGTTGG(attR3); (SEQ ID NO:11) (g)AGCCTGCTTTTTTGTACAAAGTTGG(attL1); (SEQ ID NO:12) (h)AGCCTGCTTTCTTGTACAAAGTTGG(attL2); (SEQ ID NO:13) (i)ACCCAGCTTTCTTGTACAAAGTTGG(attL3); (SEQ ID NO:14) (j)GTTCAGCTTTTTTGTACAAAGTTGG(attP1); (SEQ ID NO:15) (k)GTTCAGCTTTCTTGTACAAAGTTGG(attP2,P3); (SEQ ID NO:16)

[0173] NO:16); or a corresponding or complementary DNA or RNA sequence.

[0174] The present invention thus also provides a method for making anucleic acid molecule, comprising providing a nucleic acid moleculehaving at least one engineered recombination site comprising at leastone DNA sequence having at least 80-99% homology (or any range or valuetherein) to at least one of SEQ ID NOS:1-16, or any suitablerecombination site, or which hybridizes under stringent conditionsthereto, as known in the art.

[0175] Clearly, there are various types and permutations of suchwell-known in vitro and in vivo selection methods, each of which are notdescribed herein for the sake of brevity. However, such variations andpermutations are contemplated and considered to be the differentembodiments of the present invention.

[0176] It is important to note that as a result of the preferredembodiment-being in vitro recombination reactions, non-biologicalmolecules such as PCR products can be manipulated via the presentrecombinational cloning method. In one example, it is possible to clonelinear molecules into circular vectors.

[0177] There are a number of applications for the present invention.These uses include, but are not limited to, changing vectors, apposingpromoters with genes, constructing genes for fusion proteins, changingcopy number, changing replicons, cloning into phages, and cloning, e.g.,PCR products (with an attB site at one end and a loxP site at the otherend), genomic DNAs, and cDNAs.

[0178] The following examples are intended to further illustrate certainpreferred embodiments of the invention and are not intended to belimiting in nature.

EXAMPLES

[0179] The present recombinational cloning method accomplishes theexchange of nucleic acid segments to render something useful to theuser, such as a change of cloning vectors. These segments must beflanked on both sides by recombination signals that are in the properorientation with respect to one another. In the examples below the twoparental nucleic acid molecules (e.g., plasmids) are called the InsertDonor and the Vector Donor. The Insert Donor contains a segment thatwill become joined to a new vector contributed by the Vector Donor. Therecombination intermediate(s) that contain(s) both starting molecules iscalled the Cointegrate(s). The second recombination event produces twodaughter molecules, called the Product (the desired new clone) and theByproduct.

Buffers

[0180] Various known buffers can be used in the reactions of the presentinvention. For restriction enzymes, it is advisable to use the buffersrecommended by the manufacturer. Alternative buffers can be readilyfound in the literature or can be devised by those of ordinary skill inthe art.

[0181] Examples 1-3. One exemplary buffer for lambda integrase iscomprised of 50 mM Tris-HCl, at pH 7.5-7.8, 70 mM KCl, 5 mM spermidine,0.5 mM EDTA, and 0.25 mg/ml bovine serum albumin, and optionally, 10%glycerol.

[0182] One preferred buffer for P1 Cre recombinase is comprised of 50 mMTris-HCl at pH 7.5, 33 mM NaCl, 5 mM spermidine, and 0.5 mg/ml bovineserum alburnin.

[0183] The buffer for other site-specific recombinases which are similarto lambda Int and P1 Cre are either known in the art or can bedetermined empirically by the skilled artisans, particularly in light ofthe above-described buffers.

Example 1 Recombinational Cloning Using Cre and Cre & Int

[0184] Two pairs ofplasmids were constructed to do the in vitrorecombinational cloning method in two different ways. One pair, pEZC705and pEZC726 (FIG. 2A), was constructed with loxP and att sites, to beused with Cre and λ integrase. The other pair, pEZC602 and pEZC629 (FIG.3A), contained the loxP (wild type) site for Cre, and a second mutantlox site, loxP 511, which differs from loxP in one base (out of 34total). The minimum requirement for recombinational cloning of thepresent invention is two recombination sites in each plasmid, in generalX and Y, and X′ and Y′. Recombinational cloning takes place if either orboth types of site can recombine to form a Cointegrate (e.g. X and X′),and if either or both (but necessarily a site different from the typeforming the Cointegrate) can recombine to excise the Product andByproduct plasmids from the Cointegrate (e.g. Y and Y′). It is importantthat the recombination sites on the same plasmid do not recombine. Itwas found that the present recombinational cloning could be done withCre alone.

Cre-Only

[0185] Two plasmids were constructed to demonstrate this conception (seeFIG. 3A). pEZC629 was the Vector Donor plasmid. It contained aconstitutive drug marker (chloramphenicol resistance), an origin ofreplication, loxP and loxP 511 sites, a conditional drug marker(kanamycin resistance whose expression is controlled by theoperator/promoter of the tetracycline resistance operon of transposonTn10), and a constitutively expressed gene for the tet repressorprotein, tetR. E. coli cells containing pEZC629 were resistant tochloramphenicol at 30 μg/ml but sensitive to kanamycin at 100 μg/ml.pEZC602 was the Insert Donor plasmid, which contained a different drugmarker (ampicillin resistance), an origin, and loxP and loxP 511 sitesflanking a multiple cloning site.

[0186] This experiment was comprised of two parts as follows:

[0187] Part I: About 75 ng each of pEZC602 and pEZC629 were mixed in atotal volume of 30 μl of Cre buffer (50 mM Tris-HCl pH 7.5, 33 mM NaCl,5 mM spermidine-HCl, 500 μg/ml bovine serum albumin). Two 10 μl aliquotswere transferred to new tubes. One tube received 0.5 μl of Cre protein(approx. 4 units per μl; partially purified according to Abremski andHoess, J. Biol. Chem. 259:1509 (1984)). Both tubes were incubated at 37°C. for 30 minutes, then 70° C. for 10 minutes. Aliquots of each reactionwere diluted and transformed into DH5α. Following expression, aliquotswere plated on 30 μg/ml chloramphenicol; 100 μg/ml ampicillin plus 200μg/ml methicillin; or 100 μg/ml kanamycin. Results: See Table 1. Thereaction without Cre gave 1.11×10⁶ ampicillin resistant colonies (fromthe Insert Donor plasmid pEZC602); 7.8×10⁵ chloramphenicol resistantcolonies (from the Vector Donor plasmid pEZC629); and 140 kanamycinresistant colonies (background). The reaction with added Cre gave7.5×10⁵ ampicillin resistant colonies (from the Insert Donor plasmidpEZC602); 6.1×10⁵ chloramphenicol resistant colonies (from the VectorDonor plasmid pEZC629); and 760 kanamycin resistant colonies (mixture ofbackground colonies and colonies from the recombinational cloningProduct plasmid). Analysis: Because the number of colonies on thekanamycin plates was much higher in the presence of Cre, many or most ofthem were predicted to contain the desired Product plasmid. TABLE 1Enzyme Ampicillin Chloramphenicol Kanamycin Efficiency None 1.1 × 10⁶7.8 × 10⁵ 140 140/7.8 × 10⁵ = 0.02% Cre 7.5 × 10⁵ 6.1 × 10⁵ 760 760/6.1× 10⁵ = 0.12%

[0188] Part II: Twenty four colonies from the “+Cre” kanamycin plateswere picked and inoculated into medium containing 100 μg/ml kanamycin.Minipreps were done, and the miniprep DNAs, uncut or cut with SmaI orHindIII, were electrophoresed. Results: 19 of the 24 minipreps showedsupercoiled plasmid of the size predicted for the Product plasmid. All19 showed the predicted SmaI and HindIII restriction fragments.Analysis: The Cre only scheme was demonstrated. Specifically, it wasdetermined to have yielded about 70% (19 of 24) Product clones. Theefficiency was about 0.1% (760 kanamycin resistant clones resulted from6.1×10⁵ chloramphenicol resistant colonies).

Cre Plus Integrase

[0189] The plasmids used to demonstrate this method are exactlyanalogous to those used above, except that pEZC726, the Vector Donorplasmid, contained an attP site in place of loxP 511, and pEZC705, theInsert Donor plasmid, contained an attB site in place of loxP 511 (FIG.2A).

[0190] This experiment was comprised of three parts as follows:

[0191] Part I: About 500 ng of pEZC705 (the Insert Donor plasmid) wascut with ScaI, which linearized the plasmid within the ampicillinresistance gene. This was done because the λ integrase reaction has beenhistorically done with the attB plasmid in a linear state (H. Nash,personal communication). However, it was found later that the integrasereaction proceeds well with both plasmids supercoiled.) Then, the linearplasmid was ethanol precipitated and dissolved in 20 μl of λ integrasebuffer (50 mM Tris-HCl, about pH 7.8, 70 mM KCl, 5 mM spermidine-HCl,0.5 mM EDTA, 250 μg/ml bovine serum albumin). Also, about 500 ng of theVector Donor plasmid pEZC726 was ethanol precipitated and dissolved in20 μl λ integrase buffer. Just before use, λ integrase (2 μl, 393 μg/ml)was thawed and diluted by adding 18 μl cold λ integrase buffer. One μlIHF (integration host factor, 2.4 mg/ml, an accessory protein) wasdiluted into 150 μl cold λ integrase buffer. Aliquots (2 μl) of each DNAwere mixed with λ integrase buffer, with or without 1 μl each λintegrase and IHF, in a total of 10 μl. The mixture was incubated at 25°C. for 45 minutes, then at 70° C. for 10 minutes. Half of each reactionwas applied to an agarose gel. Results: In the presence of integrase andIHF, about 5% of the total DNA was converted to a linear Cointegrateform. Analysis. Activity of integrase and IHF was confirmed.

[0192] Part II: Three microliters of each reaction (i.e., with orwithout integrase and IHF) were diluted into 27 μl of Cre buffer(above), then each reaction was split into two 10 μl aliquots (fouraltogether). To two of these reactions, 0.5 μl of Cre protein (above)were added, and all reactions were incubated at 37° C. for 30 minutes,then at 70° C. for 10 minutes. TE buffer (90 μl; TE: 10 mM Tris-HCl, pH7.5, 1 mM EDTA) was added to each reaction, and 1 μl each wastransformed into E. coil DH5α. The transformation mixtures were platedon 100 μl g/ml ampicillin plus 200 μg/ml methicillin; 30 μg/mlchloramphenicol; or 100 μg/ml kanamycin. Results: See Table 2. TABLE 2Enzyme Ampicillin Chloramphenicol Kanamycin Efficiency None 990 20000 4 4/2 × 10⁴ = 0.02% Cre only 280 3640 0 0 Integrase* 1040 27000 9  9/2.7× 10⁴ = only 0.03% Integrase* 110 1110 76 76/1.1 × 10³ = +Cre 6.9%

[0193] Analysis: The Cre protein impaired transformation. When adjustedfor this effect, the number of kanamycin resistant colonies, compared tothe control reactions, increased more than 100 fold when both Cre andIntegrase were used. This suggests a specificity of greater than 99%.

[0194] Part III: 38 colonies were picked from the Integrase plus Creplates, miniprep DNAs were made and cut with HindIII to give diagnosticmapping information. Result: All 38 had precisely the expected fragmentsizes. Analysis: The Cre plus λ integrase method was observed to havemuch higher specificity than Cre-alone. Conclusion: The Cre plus λintegrase method was demonstrated. Efficiency and specificity were muchhigher than for Cre only.

Example 2 Using in viro Recombinational Cloning to Subclone theChloramphenicol Acetyl Transferase Gene into a Vector for Expression inEukaryotic Cells (FIG. 4A)

[0195] An Insert Donor plasmid, pEZC843, was constructed, comprising thechloramphenicol acetyl transferase gene of E. coli, cloned between loxPand attB sites such that the loxP site was positioned at the 5′-end ofthe gene (FIG. 4B). A Vector Donor plasmid, pEZC1003, was constructed,which contained the cytomegalovirus eukaryotic promoter apposed to aloxP site (FIG. 4C). One microliter aliquots of each supercoiled plasmid(about 50 ng crude miniprep DNA) were combined in a ten microliterreaction containing equal parts of lambda integrase buffer (50 mMTris-HCl, pH 7.8, 70 mM KCl, 5 mM spermidine, 0.5 mM EDTA, 0.25 mg/mlbovine serum albumin) and Cre recombinase buffer (50 mM Tris-HCl, pH7.5, 33 mM NaCl, 5 mM spermidine, 0.5 mg/ml bovine serum albumin), twounits of Cre recombinase, 16 ng integration host factor, and 32 nglambda integrase. After incubation at 30° C. for 30 minutes and 75° C.for 10 minutes, one microliter was transformed into competent E. colistrain DH5α (Life Technologies, Inc.). Aliquots of transformations werespread on agar plates containing 200 μg/ml kanamycin and incubated at37° C. overnight. An otherwise identical control reaction contained theVector Donor plasmid only. The plate receiving 10% of the controlreaction transformation gave one colony; the plate receiving 10% of therecombinational cloning reaction gave 144 colonies. These numberssuggested that greater than 99% of the recombinational cloning coloniescontained the desired product plasmid. Miniprep DNA made from sixrecombinational cloning colonies gave the predicted size plasmid (5026base pairs), CMVProd. Restriction digestion with NcoI gave the fragmentspredicted for the chloramphenicol acetyl transferase cloned downstreamof the CMV promoter for all six plasmids.

Example 3 Subcloned DNA Segments Flanked by attB Sites Without StopCodons

[0196] Part I: Background

[0197] The above examples are suitable for transcriptional fusions, inwhich transcription crosses recombination sites. However, both attR andloxP sites contain multiple stop codons on both strands, sotranslational fusions can be difficult, where the coding sequence mustcross the recombination sites, (only one reading frame is available oneach strand of loxP sites) or impossible (in attR or attL).

[0198] A principal reason for subcloning is to fuse protein domains. Forexample, fusion of the glutathione S-transferase (GST) domain to aprotein of interest allows the fusion protein to be purified by affinitychromatography on glutathione agarose (Pharmacia, Inc., 1995 catalog).If the protein of interest is fused to runs of consecutive histidines(for example His6), the fusion protein can be purified by affinitychromatography on chelating resins containing metal ions (Qiagen, Inc.).It is often desirable to compare amino terminal and carboxy terminalfusions for activity, solubility, stability, and the like.

[0199] The attB sites of the bacteriophage λ integration system wereexamined as an alternative to loxP sites, because they are small (25 bp)and have some sequence flexibility (Nash, H. A. et al., Proc. Natl. AcadSci. USA 84:4049-4053 (1987). It was not previously suggested thatmultiple mutations to remove all stop codes would result in usefulrecombination sites for recombinational subcloning.

[0200] Using standard nomenclature for site specific recombination inlambda bacteriophage (Weisber, in Lambda III, Hendrix, et al., eds.,Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1989)), thenucleotide regions that participate in the recombination reaction in anE. coli host cell are represented as follows: attP--P1--H1--P2--X--H2--C-O-C--H′--P′1--P′2--P′3--                        +attB                    --B-O-B′--             Int, IHF ↓↑ Xis, Int, IHFattR --P1--H1--P2--X--H2--C-O-B′--                        + attL                   --B-O-C--H′--P′1--P′2--P′3--,

[0201] where: O represents the 15 bp core DNA sequence found in both thephage and E. coli genomes; B and B′ represent approximately 5 basesadjacent to the core in the E. coli genome; and P1, H1, P2, X, H2, C,C′, H′, P′1, P′2, and P′3 represent known DNA sequences encoding proteinbinding domains in the bacteriophage λ genome.

[0202] The reaction is reversible in the presence of the protein Xis(excisionase); recombination between attL and attR precisely excise theλ genome from its integrated state, regenerating the circular λ genomecontaining attP and the linear E. coli genome containing attB.

[0203] Part II: Construction and Testing of Plasmids Containing Mutantatt Sites

[0204] Mutant attL and attR sites were constructed. Importantly, Landyet al. (Ann. Rev. Biochem. 58:913 (1989)) observed that deletion of theP1 and H1 domains of attP facilitated the excision reaction andeliminated the integration reaction, thereby making the excisionreaction irreversible. Therefore, as mutations were introduced in attR,the P1 and H1 domains were also deleted. attR sites in the presentexample lack the P1 and H1 regions and have the NdeI site removed (base27630 changed from C to G), and contain sequences corresponding tobacteriophage λ coordinates 27619-27738 (GenBank release 92.0, bg:LAMCG,“Complete Sequence of Bacteriophage Lambda”).

[0205] The sequence of attB produced by recombination of wild type attLand attR sites is: attBwt:    B            O            B′ 5′ AGCCTGCTTTTTTATACTAA CTTGA 3′ (SEQ. ID NO:31) 3′ TCGGA CGAAAAAATATGATT GAACT5′

[0206] The stop codons are italicized and underlined. Note thatsequences of attL, attR, and attP can be derived from the attB sequenceand the boundaries of bacteriophage λ contained within attL and attR(coordinates 27619 to 27818).

[0207] When mutant attR1 and attL1 sites were recombined the sequenceattB1 was produced (mutations in bold, large font): attB1:   B            O            B′ ^(5′ AGCCT) _(GCTTTTTT) G _(TAC) A_(AA CTTG) T ₃ (SEQ. ID NO:6) ^(3′ TCGGA) _(CGAAAAAA) C _(ATG) T_(TT GAAC) A _(5′)

[0208] Note that the four stop codons are gone.

[0209] When an additional mutation was introduced in the attR1 and attL1sequences (bold), attR2 and attL2 sites resulted. Recombination of attR2and attL2 produced the attB2 site: attB2:   B            O            B′ ^(5′ AGCCT) _(GCTTT) C _(TTGTACAAA)^(CTTGT 3′) (SEQ. ID NO:7) ^(3′ TCGGA) _(CGAAA) G _(AACATGTTT)^(GAACA 5′)

[0210] The recombination activities of the above attL and attR siteswere assayed as follows. The attB site of plasmid pEZC705 (FIG. 2B) wasreplaced with attLwt, attL1, or attL2. The attP site of plasmid pEZC726(FIG. 2C) was replaced with attRwt (lacking regions P1 and H1), attR1,or attR2. Thus, the resulting plasmids could recombine via their loxPsites, mediated by Cre, and via their attR and attL sites, mediated byInt, Xis, and IHF. Pairs of plasmids were mixed and reacted with Cre,Int, Xis, and IHF, transformed into E. coli competent cells, and platedon agar containing kanamycin. The results are presented in Table 3:TABLE 3 # of kanamycin resistant Vector donor att site Gene donor attsite colonies* arrRwt (pEZC1301) None  1 (background) ″ arrLwt(pEZC1313) 147 ″ attL1 (pEZC1317)  47 ″ attL2 (pEZC1321)  0 attR1(pEZC1305) None  1 (background) ″ attLwt (pEZC1313)  4 ″ attL1(pEZC1317) 128 ″ attL2 (pEZC1321)  0 attR2 (pEZC1309) None  9(background) ″ attLwt (pEZC1313)  0 ″ attL2 (pEZC1317)  0 ″ attL2(pEZC1321) 209

[0211] The above data show that whereas the wild type att and att1 sitesrecombine to a small extent, the att1 and att2 sites do not recombinedetectably with each other.

[0212] Part III. Recombination was demonstrated when the core region ofboth attB sites flanking the DNA segment of interest did not containstop codons. The physical state of the participating plasmids wasdiscovered to influence recombination efficiency.

[0213] The appropriate att sites were moved into pEZC705 and pEZC726 tomake the plasmids pEZC1405 (FIG. 5G) (attR1 and attR2) and pEZC1502(FIG. 5H) (attL1 and attL2). The desired DNA segment in this experimentwas a copy of the chloramphenicol resistance gene cloned between the twoattL sites of pEZC1502. Pairs of plasmids were recombined in vitro usingInt, Xis, and IHF (no Cre because no loxP sites were present). The yieldof desired kanamycin resistant colonies was determined when bothparental plasmids were circular, or when one plasmid was circular andthe other linear as presented in Table 4: TABLE 4 Kanamycin resistantVector donor¹ Gene donor¹ colonies² Circular pEZC1405 None 30 CircularpEZC1405 Circular pEZC1502 2680 Linear pEZC1405 None 90 Linear pEZC1405Circular pEZC1502 172000 Circular pEZC1405 Linear pEZC1502 73000

[0214] Analysis: Recombinational cloning using mutant attR and attLsites was confirmed. The desired DNA segment is subcloned between attBsites that do not contain any stop codons in either strand. The enhancedyield of Product DNA (when one parent was linear) was unexpected becauseof earlier observations that the excision reaction was more efficientwhen both participating molecules were supercoiled and proteins werelimiting (Nunes-Duby et al., Cell 50:779-788 (1987).

Example 4 Demonstration of Recombinational Cloning Without InvertedRepeats

[0215] Part I: Rationale

[0216] The above Example 3 showed that plasmids containing invertedrepeats of the appropriate recombination sites (for example, attL1 andattL2 in plasmid pEZC1502) (FIG. 5H) could recombine to give the desiredDNA segment flanked by attB sites without stop codons, also in invertedorientation. A concern was the in vivo and in vitro influence of theinverted repeats. For example, transcription of a desired DNA segmentflanked by attB sites in inverted orientation could yield a singlestranded RNA molecule that might form a hairpin structure, therebyinhibiting translation.

[0217] Inverted orientation of similar recombination sites can beavoided by placing the sites in direct repeat arrangement att sites. Ifparental plasmids each have a wild type attL and wild type attR site, indirect repeat the Int, Xis, and IHF proteins will simply remove the DNAsegment flanked by those sites in an intramolecular reaction. However,the mutant sites described in the above Example 3 suggested that itmight be possible to inhibit the intramolecular reaction while allowingthe intermolecular recombination to proceed as desired.

[0218] Part II: Structure ofPlasmids Without Inverted Repeats forRecombinational Cloning

[0219] The attR2 sequence in plasmid pEZC1405 (FIG. 5G) was replacedwith attL2, in the opposite orientation, to make pEZC1603 (FIG. 6A). TheattL2 sequence of pEZC1502 (FIG. 5H) was replaced with attR2, in theopposite orientation, to make pEZC1706 (FIG. 6B). Each of these plasmidscontained mutations in the core region that make intramolecularreactions between att1 and att2 cores very inefficient (see Example 3,above).

[0220] Plasmids pEZC1405, pEZC1502, pEZC1603 and pEZC1706 were purifiedon Qiagen columns (Qiagen, Inc.). Aliquots of plasmids pEZC1405 andpEZC1603 were linearized with Xba I. Aliquots of plasmids pEZC1502 andpEZC1706 were linearized with AlwN I. One hundred ng of plasmids weremixed in buffer (equal volumes of 50 mM Tris HCl pH 7.5,25 mM Tris HClpH 8.0, 70 mM KCl, 5 mM spermidine, 0.5 mM EDTA, 250 μg/ml BSA, 10%glycerol) containing Int (43.5 ng), Xis (4.3 ng) and IHF (8.1 ng) in afinal volume of 10 μl. Reactions were incubated for 45 minutes at 25°C., 10 minutes at 65° C., and 1 μl was transformed into E. coli DH5α.After expression, aliquots were spread on agar plates containing 200μg/ml kanamycin and incubated at 37° C.

[0221] Results, expressed as the number of colonies per 1 μl ofrecombination reaction are presented in Table 5: TABLE 5 Vector DonorGene Donor Colonies Predicted % product Circular 1405 — 100 — Circular1405 Circular 1502 3740 3640/3740 = 97% Linear 1405 — 90 — Linear 1405Circular 1502 172,000 171,910/172,000 = 99.9% Circular 1405 Linear 150273,000 72,900/73,000 = 99.9% Circular 1603 — 80 — Circular 1603 Circular1706 410 330/410 = 80% Linear 1603 — 270 — Linear 1603 Circular 17067000 6730/7000 = 96% Circular 1603 Linear 1706 10,800 10,530/10,800 =97%

[0222] Analysis. In all configurations, i.e., circular or linear, thepEZC1405×pEZC1502 pair (with att sites in inverted repeat configuration)was more efficient than pEZC1603×pEZC1706 pair (with att sites mutatedto avoid hairpin formation). The pEZC1603×pEZC1706 pair gave higherbackgrounds and lower efficiencies than the pEZC1405×pEZC1502 pair.While less efficient, 80% or more of the colonies from thepEZC1603×pEZC1706 reactions were expected to contain the desired plasmidproduct. Making one partner linear stimulated the reactions in allcases.

[0223] Part III: Confirmation of Product Plasmids' Structure

[0224] Six colonies each from the linear pEZC405 (FIG. 5G)×circularpEZC1502 (FIG. 5H), circular pEZC1405×linear pEZC1502, linear pEZC1603(FIG. 6A)×circular pEZC1706 (FIG. 6B), and circular pEZC1603×linearpEZC1706 reactions were picked into rich medium and miniprep DNAs wereprepared. Diagnostic cuts with Ssp I gave the predicted restrictionfragments for all 24 colonies.

[0225] Analysis. Recombination reactions between plasmids with mutantattL and attR sites on the same molecules gave the desired plasmidproducts with a high degree of specificity.

Example 5 Recombinational Cloning with a Toxic Gene

[0226] Part II: Background

[0227] Restriction enzyme Dpn I recognizes the sequence GATC and cutsthat sequence only if the A is methylated by the dam methylase. Mostcommonly used E. coli strains are dam⁺. Expression of Dpn I in dam⁺strains of E. coli is lethal because the chromosome of the cell ischopped into many pieces. However, in dam⁻ cells expression of Dpn I isinnocuous because the chromosome is immune to Dpn I cutting.

[0228] In the general recombinational cloning scheme, in which thevector donor contains two segments C and D separated by recombinationsites, selection for the desired product depends upon selection for thepresence of segment D, and the absence of segment C. In the originalExample segment D contained a drug resistance gene (Km) that wasnegatively controlled by a repressor gene found on segment C. When C waspresent, cells containing D were not resistant to kanamycin because theresistance gene was turned off.

[0229] The Dpn I gene is an example of a toxic gene that can replace therepressor gene of the above embodiment. If segment C expresses the Dpn Igene product, transforming plasmid CD into a dam⁺ host kills the cell.If segment D is transferred to a new plasmid, for example byrecombinational cloning, then selecting for the drug marker will besuccessful because the toxic gene is no longer present.

[0230] Part II: Construction of a Vector Donor Using Dpn I as a ToxicGene

[0231] The gene encoding Dpn I endonuclease was amplified by PCR usingprimers 5′CCA CCA CAA ACG CGT CCA TGG AAT TAC ACT TTA ATT TAG3′(SEQ. IDNO: 17) and 5′CCA CCA CAA GTC GAC GCA TGC CGA CAG CCT TCC AAA TGT3′(SEQ. ID NO:18) and a plasmid containing the Dpn I gene (derived fromplasmids obtained from Sanford A. Lacks, Brookhaven National Laboratory,Upton, N.Y.; also available from American Type Culture Collection asATCC 67494) as the template.

[0232] Additional mutations were introduced into the B and B′ regions ofattL and attR, respectively, by amplifying existing attL and attRdomains with primers containing the desired base changes. Recombinationof the mutant attL3 (made with oligo Xis115) and attR3 (made with oligoXis112) yielded attB3 with the following sequence (differences fromattB1 in bold):   B          O         B′ _(A) C _(CC) A _(GCTTT) C_(TTGTACAAA) G _(T) G _(GT) (SEQ. ID NO:8) _(T) G _(GG) T _(CGAAA) G_(AACATGTTT) C _(A) C _(CA)

[0233] The attL3 sequence was cloned in place of attL2 of an existingGene Donor plasmid to give the plasmid pEZC2901 (FIG. 7A). The attR3sequence was cloned in place of attR2 in an existing Vector Donorplasmid to give plasmid pEZC2913 (FIG. 7B) Dpn I gene was cloned intoplasmid pEZC2913 to replace the tet repressor gene. The resulting VectorDonor plasmid was named pEZC3101 (FIG. 7C). When pEZC3101 wastransformed into the dam⁻ strain SCS110 (Stratagene), hundreds ofcolonies resulted. When the same plasmid was transformed into the dam+strain DH5α, only one colony was produced, even though the DH5α cellswere about 20 fold more competent than the SCS110 cells. When a relatedplasmid that did not contain the Dpn I gene was transformed into thesame two cell lines, 28 colonies were produced from the SCS110 cells,while 448 colonies resulted from the DH5α cells. This is evidence thatthe Dpn I gene is being expressed on plasmid pEZC3101 (FIG. 7C), andthat it is killing the dam⁺ DH5α cells but not the dam⁻ SCS110 cells.

[0234] Part III: Demonstration of Recombinational Cloning Using Dpn ISelection

[0235] A pair of plasrnids was used to demonstrate recombinationalcloning with selection for product dependent upon the toxic gene Dpn I.Plasmid pEZC3101 (FIG. 7C) was linearized with Mlu I and reacted withcircular plasmid pEZC2901 (FIG. 7A). A second pair of plasmids usingselection based on control of drug resistance by a repressor gene wasused as a control: plasmid pEZC1802 (FIG. 7D) was linearized with Xba Iand reacted with circular plasmid pEZC1502 (FIG. 5H). Eight microliterreactions containing the same buffer and proteins Xis, Int, and IHF asin previous examples were incubated for 45 minutes at 25° C., then 10minutes at 75° C., and 1 μl aliquots were transformed into DH5α (i.e.,dam+) competent cells, as presented in Table 6. TABLE 6 Reaction Basisof # Vector donor selection Gene donor Colonies 1 pEZC3101/Mlu Dpn Itoxicity — 3 2 pEZC3101/Mlu Dpn I toxicity Circular 4000 pEZC2901 3pEZC1802/Xba Tet repressor — 0 4 pEZC1802/Xba Tet repressor Circular12100 pEZC1502

[0236] Miniprep DNAs were prepared from four colonies from reaction #2,and cut with restriction enzyme Ssp I. All gave the predicted fragments.

[0237] Analysis: Subcloning using selection with a toxic gene wasdemonstrated. Plasmids of the predicted structure were produced.

Example 6 Cloning of Genes with Uracil DNA Glycosylase and Subcloning ofthe Genes with Recombinational Cloning to Make Fusion Proteins

[0238] Part I: Converting an Existing Expression Vector to a VectorDonor for Recombinational Cloning

[0239] A cassette useful for converting existing vectors into functionalVector Donors was made as follows. Plasmid pEZC3101 (FIG. 7C) wasdigested with Apa I and Kpn I, treated with T4 DNA polymerase and dNTPsto render the ends blunt, further digested with Sma I, Hpa I, and AlwN Ito render the undesirable DNA fragments small, and the 2.6 kb cassettecontaining the attR1-Cm^(R)-Dpn I-attR-3 domains was gel purified. Theconcentration of the purified cassette was estimated to be about 75 ngDNA/μl.

[0240] Plasmid pGEX-2TK (FIG. 8A) (Pharmacia) allows fusions between theprotein glutathione S transferase and any second coding sequence thatcan be inserted in its multiple cloning site. pGEX-2TK DNA was digestedwith Sma I and treated with alkaline phosphatase. About 75 ng of theabove purified DNA cassette was ligated with about 100 ng of thepGEX-2TK vector for 2.5 hours in a 5 μl ligation, then 1 μl wastransformed into competent BRL 3056 cells (a dam⁻ derivative of DH10B;dam⁻ strains commercially available include DMI from Life Technologies,Inc., and SCS 110 from Stratagene). Aliquots of the transformationmixture were plated on LB agar containing 100 μg/ml ampicillin(resistance gene present on pGEX-2TK) and 30 μg/ml chloramphenicol(resistance gene present on the DNA cassette). Colonies were picked andminiprep DNAs were made. The orientation of the cassette in pGEX-2TK wasdetermined by diagnostic cuts with EcoR I. A plasmid with the desiredorientation was named pEZC3501 (FIG. 8B).

[0241] Part II: Cloning Reporter Genes Into an Recombinational CloningGene Donor Plasmid in Three Reading Frames

[0242] Uracil DNA glycosylase (UDG) cloning is a method for cloning PCRamplification products into cloning vectors (U.S. Pat. No. 5,334,515,entirely incorporated herein by reference). Briefly, PCR amplificationof the desired DNA segment is performed with primers that contain uracilbases in place of thymidine bases in their 5′ ends. When such PCRproducts are incubated with the enzyme UDG, the uracil bases arespecifically removed. The loss of these bases weakens base pairing inthe ends of the PCR product DNA, and when incubated at a suitabletemperature (e.g., 37° C.), the ends of such products are largely singlestranded. If such incubations are done in the presence of linear cloningvectors containing protruding 3′ tails that are complementary to the 3′ends of the PCR products, base pairing efficiently anneals the PCRproducts to the cloning vector. When the annealed product is introducedinto E. coli cells by transformation, in vivo processes efficientlyconvert it into a recombinant plasmid.

[0243] UDG cloning vectors that enable cloning of any PCR product in allthree reading frames were prepared from pEZC3201 (FIG. 8K) as follows.Eight oligonucleotides were obtained from Life Technologies, Inc. (allwritten 5′→3′: rf1 top (GGCC GAT TAC GAT ATC CCA ACG ACC GAA AAC CTG.TATTTT CAG GGT) (SEQ. ID NO:19), rf1 bottom (CAG GTT TTC GGT CGT TGG GATATC GTA ATC)(SEQ. ID NO:20), rf2 top (GGCCA GAT TAC GAT ATC CCA ACG ACCGAA AAC CTG TAT TTT CAG GGT)(SEQ. ID NO:21), rf2 bottom (CAG GTT TTC GGTCGT TGG GAT ATC GTA ATC T)(SEQ. ID NO:22), rf3 top (GGCCAA GAT TAC GATATC CCA ACG ACC GAA AAC CTG TAT TTT CAG GGT)(SEQ. ID NO:23), rf3 bottom(CAG GTT TTC GGT CGT TGG GAT ATC GTA ATC TT(SEQ. ID NO:24), carboxy top(ACC GTT TAC GTG GAC)(SEQ. ID NO:25) and carboxy bottom (TCGA GTC CACGTA AAC GGT TCC CAC TTA TTA)(SEQ. ID NO:26). The rf1, 2, and 3 topstrands and the carboxy bottom strand were phosphorylated on their 5′ends with T4 polynucleotide kinase, and then the complementary strandsof each pair were hybridized. Plasmid pEZC3201 (FIG. 8K) was cut withNot I and Sal I, and aliquots of cut plasmid were mixed with thecarboxy-oligo duplex (Sal I end) and either the rf1, rf2, or rf3duplexes (Not I ends) (10 μg cut plasmid (about 5 pmol) mixed with 250pmol carboxy oligo duplex, split into three 20 μl volumes, added 5 μl(250 pmol) of rf1, rf2, or rf3 duplex and 2 μl=2 units T4 DNA ligase toeach reaction). After 90 minutes of ligation at room temperature, eachreaction was applied to a preparative agarose gel and the 2.1 kb vectorbands were eluted and dissolved in 50 μl of TE.

[0244] Part III: PCR of CAT and phoA Genes

[0245] Primers were obtained from Life Technologies, Inc., to amplifythe chloramphenicol acetyl transferase (CAT) gene from plasmid pACYC184,and phoA, the alkaline phosphatase gene from E. coli. The primers had12-base 5′ extensions containing uracil bases, so that treatment of PCRproducts with uracil DNA glycosylase (UDG) would weaken base pairing ateach end of the DNAs and allow the 3′ strands to anneal with theprotruding 3′ ends of the rf1, 2, and 3 vectors described above. Thesequences of the primers (all written 5′→3′) were: CAT left, UAU UUU CAGGGU ATG GAG AAA AAA ATC ACT GGA TAT ACC (SEQ. ID NO:27); CAT right, UCCCAC UUA UUA CGC CCC GCC CTG CCA CTC ATC (SEQ. ID NO:28); phoA left, UAUUUU CAG GGU ATG CCT GTT CTG GAA AAC CGG (SEQ. ID NO:29); and phoA right,UCC CAC UUA UUA TTT CAG CCC CAG GGC GGC TTT C (SEQ. ID NO:30). Theprimers were then used for PCR reactions using known method steps (see,e.g., U.S. Pat. No. 5,334,515, entirely incorporated herein byreference), and the polymerase chain reaction amplification productsobtained with these primers comprised the CAT or phoA genes with theinitiating ATGs but without any transcriptional signals. In addition,the uracil-containing sequences on the amino termini encoded thecleavage site for TEV protease (Life Technologies, Inc.), and those onthe carboxy terminal encoded consecutive TAA nonsense codons.

[0246] Unpurified PCR products (about 30 ng) were mixed with the gelpurified, linear rf1, rf2, or rf3 cloning vectors (about 50 ng) in a 10μl reaction containing 1X REact 4 buffer (LTI) and 1 unit UDG (LTI).After 30 minutes at 37° C., 1 μl aliquots of each reaction weretransformed into competent E. coli DH5α cells (LTI) and plated on agarcontaining 50 μg/ml kanamycin. Colonies were picked and analysis ofminiprep DNA showed that the CAT gene had been cloned in reading frame 1(pEZC3601)(FIG. 8C), reading frame 2 (pEZC3609)(FIG. 8D) and readingframe 3 (pEZC3617)(FIG. 8E), and that the phoA gene had been cloned inreading frame 1 (pEZC3606)(FIG. 8F), reading frame 2 (pEZC3613)(FIG. 8G)and reading frame 3 (pEZC3621)(FIG. 8H).

[0247] Part IV: Subcloning of CAT or phoA from UDG Cloning Vectors intoa GST Fusion Vector

[0248] Plasmids encoding fusions between GST and either CAT or phoA inall three reading frames were constructed by recombinational cloning asfollows. Miniprep DNA of GST vector donor pEZC3501(FIG. 8B) (derivedfrom Pharmacia plasmid pGEX-2TK as described above) was linearized withCla I. About 5 ng of vector donor were mixed with about 10 ng each ofthe appropriate circular gene donor vectors containing CAT or phoA in 8μl reactions containing buffer and recombination proteins Int, Xis, andIHF (above). After incubation, 1 μl of each reaction was transformedinto E. coli strain DH5α and plated on ampicillin, as presented in Table7. TABLE 7 DNA Colonies (10% of each transformation) Linear vector donor(pEZC3501/ 0 Cla) Vector donor + CAT rf1 110 Vector donor + CAT rf2 71Vector donor + CAT rf3 148 Vector donor + phoA rf1 121 Vector donor +phoA rf2 128 Vector donor + phoA rf3 31

[0249] Part V: Expression of Fusion Proteins

[0250] Two colonies from each transformation were picked into 2 ml ofrich medium (CircleGrow, Bio101 Inc.) in 17×100 mm plastic tubes (Falcon2059, Becton Dickinson) containing 100 μg/ml ampicillin and shakenvigorously for about 4 hours at 37° C., at which time the cultures werevisibly turbid. One ml of each culture was transferred to a new tubecontaining 10 μl of 10% (w/v) IPTG to induce expression of GST. After 2hours additional incubation, all cultures had about the same turbidity;the A600 of one culture was 1.5. Cells from 0.35 ml each culture wereharvested and treated with sample buffer (containing SDS andβ-mercaptoethanol) and aliquots equivalent to about 0.15 A600 units ofcells were applied to a Novex 4-20% gradient polyacrylamide gel.Following electrophoresis the gel was stained with Coomassie blue.

[0251] Results: Enhanced expression of single protein bands was seen forall 12 cultures. The observed sizes of these proteins correlated wellwith the sizes predicted for GST being fused (through attB recombinationsites without stop codons) to CAT or phoA in three reading frames: CATrf1=269 amino acids; CAT rf2=303 amino acids; CAT rf3=478 amino acids;phoA rf1=282 amino acids; phoA rf2=280 amino acids; and phoA rf3=705amino acids.

[0252] Analysis: Both CAT and phoA genes were subcloned into a GSTfusion vector in all three reading frames, and expression of the sixfusion proteins was demonstrated.

[0253] While the foregoing invention has been described in some detailfor purposes of clarity and understanding, it will be appreciated by oneskilled in the art from a reading of this disclosure that variouschanges in form and detail can be made without departing from the truescope of the invention and appended claims. All patents and publicationscited herein are entirely incorporated herein by reference.

1 35 25 base pairs nucleic acid both both cDNA 1 RKYCWGCTTT YKTRTACNAASTSGB 25 25 base pairs nucleic acid both both cDNA 2 AGCCWGCTTTYKTRTACNAA CTSGB 25 25 base pairs nucleic acid both both cDNA 3GTTCAGCTTT CKTRTACNAA CTSGB 25 25 base pairs nucleic acid both both cDNA4 AGCCWGCTTT CKTRTACNAA GTSGB 25 25 base pairs nucleic acid both bothcDNA 5 GTTCAGCTTT YKTRTACNAA GTSGB 25 25 base pairs nucleic acid bothboth cDNA 6 AGCCTGCTTT TTTGTACAAA CTTGT 25 25 base pairs nucleic acidboth both cDNA 7 AGCCTGCTTT CTTGTACAAA CTTGT 25 25 base pairs nucleicacid both both cDNA 8 ACCCAGCTTT CTTGTACAAA CTTGT 25 25 base pairsnucleic acid both both cDNA 9 GTTCAGCTTT TTTGTACAAA CTTGT 25 25 basepairs nucleic acid both both cDNA 10 GTTCAGCTTT CTTGTACAAA CTTGT 25 25base pairs nucleic acid both both cDNA 11 GTTCAGCTTT CTTGTACAAA GTTGG 2525 base pairs nucleic acid both both cDNA 12 AGCCTGCTTT TTTGTACAAA GTTGG25 25 base pairs nucleic acid both both cDNA 13 AGCCTGCTTT CTTGTACAAAGTTGG 25 25 base pairs nucleic acid both both cDNA 14 ACCCAGCTTTCTTGTACAAA GTTGG 25 25 base pairs nucleic acid both both cDNA 15GTTCAGCTTT TTTGTACAAA GTTGG 25 25 base pairs nucleic acid both both cDNA16 GTTCAGCTTT CTTGTACAAA GTTGG 25 39 base pairs nucleic acid both bothcDNA 17 CCACCACAAA CGCGTCCATG GAATTACACT TTAATTTAG 39 39 base pairsnucleic acid both both cDNA 18 CCACCACAAG TCGACGCATG CCGACAGCCTTCCAAATGT 39 46 base pairs nucleic acid both both cDNA 19 GGCCGATTACGATATCCCAA CGACCGAAAA CCTGTATTTT CAGGGT 46 30 base pairs nucleic acidboth both cDNA 20 CAGGTTTTCG GTCGTTGGGA TATCGTAATC 30 47 base pairsnucleic acid both both cDNA 21 GGCCAGATTA CGATATCCCA ACGACCGAAAACCTGTATTT TCAGGGT 47 31 base pairs nucleic acid both both cDNA 22CAGGTTTTCG GTCGTTGGGA TATCGTAATC T 31 48 base pairs nucleic acid bothboth cDNA 23 GGCCAAGATT ACGATATCCC AACGACCGAA AACCTGTATT TTCAGGGT 48 32base pairs nucleic acid both both cDNA 24 CAGGTTTTCG GTCGTTGGGATATCGTAATC TT 32 15 base pairs nucleic acid both both cDNA 25 ACCGTTTACGTGGAC 15 31 base pairs nucleic acid both both cDNA 26 TCGAGTCCACGTAAACGGTT CCCACTTATT A 31 39 base pairs nucleic acid both both cDNA 27UAUUUUCAGG GUATGGAGAA AAAAATCACT GGATATACC 39 33 base pairs nucleic acidboth both cDNA 28 UCCCACUUAU UACGCCCCGC CCTGCCACTC ATC 33 33 base pairsnucleic acid both both cDNA 29 UAUUUUCAGG GUATGCCTGT TCTGGAAAAC CGG 3334 base pairs nucleic acid both both cDNA 30 UCCCACUUAU UATTTCAGCCCCAGGGCGGC TTTC 34 25 base pairs nucleic acid both both cDNA 31AGCCTGCTTT TTTATACTAA CTTGA 25 25 base pairs nucleic acid both both cDNA32 TCAAGTTAGT ATAAAAAAGC AGGCT 25 25 base pairs nucleic acid both bothcDNA 33 ACAAGTTTGT ACAAAAAAGC AGGCT 25 25 base pairs nucleic acid bothboth cDNA 34 ACAAGTTTGT ACAAGAAAGC AGGCT 25 25 base pairs nucleic acidboth both cDNA 35 ACCACTTTGT ACAAGAAAGC TGGGT 25

What is claimed is:
 1. A Vector Donor DNA molecule comprising a firstDNA segment and a second DNA segment, said first or second DNA segmentcontaining at least one Selectable marker, wherein the first and secondsegments are separated either by, (i) in a circular Vector Donor, afirst and a second recombination site, or (ii) in a linear Vector Donor,at least a first recombination site, wherein each pair of flankingrecombination sites are engineered and do not recombine with each other.2. A Vector Donor DNA molecule according to claim 1, wherein theSelectable marker is at least one DNA segment selected from the groupconsisting of: (i) a DNA segment that encodes a product that providesresistance against otherwise toxic compounds; (ii) a DNA segment thatencodes a product that is otherwise lacking in the recipient cell; (iii)a DNA segment that encodes a product that suppresses the activity of agene product; (iv) a DNA segment that encodes a product that can bereadily identified; (v) a DNA segment that encodes a product that isdetrimental to cell survival and/or function; (vi) a DNA segment thatinhibits the activity of any of the DNA segments of (i)-(v) above; (vii)a DNA segment that binds a product that modifies a substrate; (viii) aDNA segment that provides for isolation of a desired molecule; and (ix)a DNA segment that encodes a specific nucleotide sequence which can beotherwise non-functional; and (x) a DNA segment that, when absent,directly or indirectly confers sensitivity to particular compounds.
 3. AVector Donor DNA according to claim 2, wherein said Selectable marker isat least one selected from the group consisting of an antibioticresistance gene, a tRNA gene, an auxotrophic marker, a toxic gene, aphenotypic marker, an antisense oligonucleotide; a restrictionendonuclease; a restriction endonuclease cleavage site, an enzymecleavage site, a protein binding site; and a sequence complementary PCRprimer.
 4. A Vector Donor DNA molecule according to claim 1, whereinsaid Selectable marker comprises at least one inactive fragment of aSelectable marker, wherein the inactive fragment is capable ofreconstituting a functional Selectable marker when recombined acrosssaid first or second recombination site with a further DNA segmentcomprising another inactive fragment of the Selectable marker.
 5. AnInsert Donor DNA molecule, comprising a desired DNA segment flanked by afirst recombination site and a second recombination site, wherein thefirst and second recombination sites are engineered and do not recombinewith each other.
 6. An Insert Donor DNA molecule according to claim 5,wherein said desired DNA segment codes for at least one selected fromthe group consisting of a cloning site, a restriction site, a promoter,an operon, an origin of replication, a functional DNA, an antisense RNA,a PCR fragment, a protein or a protein fragment.
 7. A kit comprising acontainer being compartmentalized to receive in close confinementtherein at one compartment, wherein a first compartment contains aVector Donor DNA molecule comprising a first DNA segment and a secondDNA segment, said first or second DNA segment containing at least oneSelectable marker, wherein the first and second segments are flankedeither by, (i) in a circular Vector Donor, a first and a secondrecombination site, or (ii) in a linear Vector Donor, a firstrecombination site, wherein each pair of flanking recombination sitesare engineered and do not recombine with each other.
 8. A kit accordingto claim 7, further comprising a second compartment containing an InsertDonor DNA molecule comprising a desired DNA segment flanked by a firstrecombination site and a second recombination site, wherein the firstand second recombination sites are engineered and do not recombine witheach other.
 9. A kit according to claim 7, further comprising anadditional compartment containing at least one recombination proteincapable of recombining a DNA segment comprising at least one of saidrecombination sites.
 10. A nucleic acid molecule, comprising at leastone DNA segment having at least two recombination sites flanking aSelectable marker or a desired DNA segment, wherein at least one of saidrecombination sites comprises a core region having at least oneengineered mutation that enhances recombination in vitro in theformation of a Cointegrate DNA or a Product DNA.
 11. A nucleic acidmolecule according to claim 10, wherein said mutation confers at leastone enhancement of said recombination, said enhancement selected fromthe group consisting of substantially (i) favoring excisiverecombination; (ii) favoring integrative recombination; (iii) relievingthe requirement for host factors; (iv) increasing the efficiency of saidCointegrate DNA or Product DNA formation;(v) increasing the specificityof said Cointegrate DNA or Product DNA formation; and contributesdesirable attributes to the Product DNA.
 12. A nucleic acid moleculeaccording to claim 10, wherein said recombination site is derived fromat least one recombination site selected from the group consisting ofattB, attP, attL and attR.
 13. A nucleic acid molecule according toclaim 11, wherein said att site is selected from the groups consistingof att1, att2 and att3.
 14. A nucleic acid according to claim 10,wherein said core region comprises a DNA sequence selected from thegroup consisting of: (a) RKYCWGCTTTYKTRTACNAASTSGB(m-att) (SEQ ID NO:1);(b) AGCCWGCTTTYKTRTACNAACTSGB (m-attB) (SEQ ID NO:2); (c)GTTCAGCTTTCKTRTACNAACTSGB (m-attR) (SEQ ID NO:3); (d)AGCCWGCTTTCKTRTACNAAGTSGB (m-attL) (SEQ ID NO:4); (e)GTTCAGCTTTYKTRTACNAAGTSGB(m-attP1) (SEQ ID NO:5); and a corresponding orcomplementary DNA or RNA sequence, wherein R=A or G; K=G or T/U; Y=C orT/U; W=A or T/U; N=A or C or G or T/U; S=Cor G; and B=C or G or T/U. 15.A nucleic acid according to claim 14, wherein said core region comprisesa DNA sequence selected from the group consisting of: (a)AGCCTGCTTTTTTGTACAAACTTGT(attB1); (SEQ ID NO:6) (b)AGCCTGCTTTCTTGTACAAACTTGT(attB2); (SEQ ID NO:7) (c)ACCCAGCTTTCTTGTACAAACTTGT(attB3); (SEQ ID NO:8) (d)GTTCAGCTTTTTTGTACAAACTTGT(attR1); (SEQ ID NO:9) (e)GTTCAGCTTTTTTGTACAAACTTGT(attR2); (SEQ ID NO:10) (f)GTTCAGCTTTCTTGTACAAAGTTGG(attR3); (SEQ ID NO:11) (g)AGCCTGCTTTTTTGTACAAAGTTGG(attL1); (SEQ ID NO:12) (h)AGCCTGCTTTCTTGTACAAAGTTGG(attL2); (SEQ ID NO:13) (i)ACCCAGCTTTCTTGTACAAAGTTGG(attL3); (SEQ ID NO:14) (j)GTTCAGCTTTTTTGTACAAAGTTGG(attP1); (SEQ ID NO:15) (k)GTTCAGCTTTCTTGTACAAAGTTGG(attP2,P3); (SEQ ID NO:16)

and a corresponding or complementary DNA or RNA sequence.
 16. A methodfor making a nucleic acid molecule, comprising providing a nucleic acidmolecule having at least one engineered recombination site comprising atleast one DNA sequence having at least 90% homology to at least one ofSEQ ID NOS:1-16.
 17. A nucleic acid molecule provided by a methodaccording to claim
 10. 18. A composition, comprising a nucleic acidmolecule according to claim
 10. 19. A kit, comprising a container beingcompartmentalized to receive in close confinement therein at least onecompartment, wherein a first compartment contains a compositionaccording to claim
 18. 20. A kit according to claim 19, firthercomprising a second compartment having at least one recombinationprotein that recognizes said recombination site.
 21. A kit comprising acontainer being compartmentalized to receive in close confinementtherein at least one recombination protein in isolated form, useful fora method according to claim
 22. 22. A method of making a Cointegrate DNAmolecule, comprising combining in vitro: (i) an Insert Donor DNAmolecule, comprising a desired DNA segment flanked by a firstrecombination site and a second recombination site, wherein the firstand second recombination sites do not recombine with each other; (ii) aVector Donor DNA molecule containing a third recombination site and afourth recombination site, wherein the third and fourth recombinationsites do not recombine with each other; and (iii) at least one sitespecific recombination protein capable of recombining said first andthird recombinational sites said second and fourth recombinationalsites; thereby allowing recombination to occur, so as to produce aCointegrate DNA molecule comprising said first and third or said secondand fourth recombination sites.
 23. A method according to claim 22,wherein a Product DNA molecule is produced from said Cointegrate DNA byrecombining at least one of (i) said first and third, or (ii) saidsecond and fourth, recombination sites, said Product DNA comprising saiddesired DNA segment.
 24. A method according to claim 23, wherein saidmethod also produces a Byproduct DNA molecule.
 25. A method according toclaim 23, further comprising selecting for the Product DNA molecule. 26.A method as claimed in claim 22, wherein the Vector Donor DNA moleculecomprises a vector segment flanked by said third and the fourthrecombination sites.
 27. A method as claimed in claim 22, wherein theVector Donor DNA molecule further comprises (a) a toxic gene and (b) aSelectable marker, wherein the toxic gene and the Selectable marker areon different DNA segments, the DNA segments being separated either by(i) in a circular DNA molecule, two recombination sites, or (ii) in alinear DNA molecule, one recombination site.
 28. A method as claimed inclaim 22, wherein the Vector Donor DNA molecule further comprises (a) arepression cassette and (b) a Selectable marker, repressed by therepressor of the repression cassette, and wherein the Selectable markerand the repression cassette are on different DNA segments, the DNAsegments being separated either by, (i) in a circular DNA molecule, tworecombination sites, or (ii) in a linear DNA molecule, one recombinationsite.
 29. A method as claimed in claim 22, wherein at least one of theInsert Donor DNA molecule and the Vector Donor DNA molecule is acircular DNA molecule.
 30. A method as claimed in claim 22, wherein atleast one of the Insert Donor DNA molecule and the Vector Donor DNAmolecule is a linear DNA molecule.
 31. A method as claimed in claim 22,wherein the selecting step is carried out in vitro or in vivo.
 32. Amethod as claimed in claim 22, wherein said recombination proteincomprises at least a first recombination protein and a secondrecombination protein, said second recombination protein being differentfrom said first recombination protein.
 33. A method as claimed in claim22, wherein said recombination protein is Int.
 34. A method as claimedin claim 22, wherein the at least one recombination protein is selectedfrom (i) Int and IHF and (ii) Int, Xis, and IHF.